Mitochondrial reactive oxygen species perturb AKT/cyclin D1 cell cycle signaling via oxidative inactivation of PP2A in lowdose irradiated human fibroblasts

Mitochondrial reactive oxygen species impact human fibroblast responses to protracted γ-ray exposures

Purpose: Continuous exposure to ionizing radiation at a low dose rate poses significant health risks to humans on deep space missions, prompting the need for mechanistic studies to identify countermeasures against its deleterious effects. Mitochondria are a major subcellular locus of radiogenic injury, and may trigger secondary cellular responses through the production of reactive oxygen species (mtROS) with broader biological implications. Methods and Materials: To determine the contribution of mtROS to radiation-induced cellular responses, we investigated the impacts of protracted γ-ray exposures (IR; 1.1 Gy delivered at 0.16 mGy/min continuously over 5 days) on mitochondrial function, gene expression, and the protein secretome of human HCA2-hTERT fibroblasts in the presence and absence of a mitochondria-specific antioxidant mitoTEMPO (MT; 5 µM). Results: IR increased fibroblast mitochondrial oxygen consumption (JO2) and H2O2 release rates (JH2O2) under energized conditions, which corresponded to higher protein expression of NADPH Oxidase (NOX) 1, NOX4, and nuclear DNA-encoded subunits of respiratory chain Complexes I and III, but depleted mtDNA transcripts encoding subunits of the same complexes. This was associated with activation of gene programs related to DNA repair, oxidative stress, and protein ubiquination, all of which were attenuated by MT treatment along with radiation-induced increases in JO2 and JH2O2. IR also increased secreted levels of interleukin-8 and Type I collagens, while decreasing Type VI collagens and enzymes that coordinate assembly and remodeling of the extracellular matrix. MT treatment attenuated many of these effects while augmenting others, revealing complex effects of mtROS in fibroblast responses to IR. Conclusion: These results implicate mtROS production in fibroblast responses to protracted radiation exposure, and suggest potentially protective effects of mitochondrial-targeted antioxidants against radiogenic tissue injury in vivo.

Low-Dose Non-Targeted Effects and Mitochondrial Control

Non-targeted effects (NTE) have been generally regarded as a low-dose ionizing radiation (IR) phenomenon. Recently, regarding long distant abscopal effects have also been observed at high doses of IR) relevant to antitumor radiation therapy. IR is inducing NTE involving intracellular and extracellular signaling, which may lead to short-ranging bystander effects and distant long-ranging extracellular signaling abscopal effects. Internal and "spontaneous" cellular stress is mostly due to metabolic oxidative stress involving mitochondrial energy production (ATP) through oxidative phosphorylation and/or anaerobic pathways accompanied by the leakage of O₂^- and other radicals from mitochondria during normal or increased cellular energy requirements or to mitochondrial dysfunction. Among external stressors, ionizing radiation (IR) has been shown to very rapidly perturb mitochondrial functions, leading to increased energy supply demands and to ROS/NOS production. Depending on the dose, this affects all types of cell constituents, including DNA, RNA, amino acids, proteins, and membranes, perturbing normal inner cell organization and function, and forcing cells to reorganize the intracellular metabolism and the network of organelles. The reorganization implies intracellular cytoplasmic-nuclear shuttling of important proteins, activation of autophagy, and mitophagy, as well as induction of cell cycle arrest, DNA repair, apoptosis, and senescence. It also includes reprogramming of mitochondrial metabolism as well as genetic and epigenetic control of the expression of genes and proteins in order to ensure cell and tissue survival. At low doses of IR, directly irradiated cells may already exert non-targeted effects (NTE) involving the release of molecular mediators, such as radicals, cytokines, DNA fragments, small RNAs, and proteins (sometimes in the form of extracellular vehicles or exosomes), which can induce damage of unirradiated neighboring bystander or distant (abscopal) cells as well as immune responses. Such non-targeted effects (NTE) are contributing to low-dose phenomena, such as hormesis, adaptive responses, low-dose hypersensitivity, and genomic instability, and they are also promoting suppression and/or activation of immune cells. All of these are parts of the main defense systems of cells and tissues, including IR-induced innate and adaptive immune responses. The present review is focused on the prominent role of mitochondria in these processes, which are determinants of cell survival and anti-tumor RT.

NNAT is a novel mediator of oxidative stress that suppresses ER + breast cancer

BACKGROUND

Neuronatin (NNAT) was recently identified as a novel mediator of estrogen receptor-positive (ER+) breast cancer cell proliferation and migration, which correlated with decreased tumorigenic potential and prolonged patient survival. However, despite these observations, the molecular and pathophysiological role(s) of NNAT in ER + breast cancer remains unclear. Based on high protein homology with phospholamban, we hypothesized that NNAT mediates the homeostasis of intracellular calcium [Ca2+]i levels and endoplasmic reticulum (EndoR) function, which is frequently disrupted in ER + breast cancer and other malignancies.

METHODS

To evaluate the role of NNAT on [Ca2+]i homeostasis, we used a combination of bioinformatics, gene expression and promoter activity assays, CRISPR gene manipulation, pharmacological tools and confocal imaging to characterize the association between ROS, NNAT and calcium signaling.

RESULTS

Our data indicate that NNAT localizes predominantly to EndoR and lysosome, and genetic manipulation of NNAT levels demonstrated that NNAT modulates [Ca2+]i influx and maintains Ca2+ homeostasis. Pharmacological inhibition of calcium channels revealed that NNAT regulates [Ca2+]i levels in breast cancer cells through the interaction with ORAI but not the TRPC signaling cascade. Furthermore, NNAT is transcriptionally regulated by NRF1, PPARα, and PPARγ and is strongly upregulated by oxidative stress via the ROS and PPAR signaling cascades.

CONCLUSION

Collectively, these data suggest that NNAT expression is mediated by oxidative stress and acts as a regulator of Ca2+ homeostasis to impact ER + breast cancer proliferation, thus providing a molecular link between the longstanding observation that is accumulating ROS and altered Ca2+ signaling are key oncogenic drivers of cancer.

The DNA damage response to radiological imaging: from ROS and γH2AX foci induction to gene expression responses in vivo

Candidate ionising radiation exposure biomarkers must be validated in humans exposed in vivo. Blood from patients undergoing positron emission tomography-computed tomography scan (PET-CT) and skeletal scintigraphy (scintigraphy) was drawn before (0 h) and after (2 h) the procedure for correlation analyses of the response of selected biomarkers with radiation dose and other available patient information. FDXR, CDKN1A, BBC3, GADD45A, XPC, and MDM2 expression was determined by qRT-PCR, DNA damage (γH2AX) by flow cytometry, and reactive oxygen species (ROS) levels by flow cytometry using the 2', 7'-dichlorofluorescein diacetate test in peripheral blood mononuclear cells (PBMC). For ROS experiments, 0- and 2-h samples were additionally exposed to UVA to determine whether diagnostic irradiation conditioned the response to further oxidative insult. With some exceptions, radiological imaging induced weak γH2AX foci, ROS and gene expression fold changes, the latter with good coherence across genes within a patient. Diagnostic imaging did not influence oxidative stress in PBMC successively exposed to UVA. Correlation analyses with patient characteristics led to low correlation coefficient values. γH2AX fold change, which correlated positively with gene expression, presented a weak positive correlation with injected activity, indicating a radiation-induced subtle increase in DNA damage and subsequent activation of the DNA damage response pathway. The exposure discrimination potential of these biomarkers in the absence of control samples as frequently demanded in radiological emergencies, was assessed using raw data. These results suggest that the variability of the response in heterogeneous populations might complicate identifying individuals exposed to low radiation doses.

Mitochondrial Signaling Pathways Associated with DNA Damage Responses

Under physiological and stress conditions, mitochondria act as a signaling platform to initiate biological events, establishing communication from the mitochondria to the rest of the cell. Mitochondrial adenosine triphosphate (ATP), reactive oxygen species, cytochrome C, and damage-associated molecular patterns act as messengers in metabolism, oxidative stress response, bystander response, apoptosis, cellular senescence, and inflammation response. In this review paper, the mitochondrial signaling in response to DNA damage was summarized. Mitochondrial clearance via fusion, fission, and mitophagy regulates mitochondrial quality control under oxidative stress conditions. On the other hand, damaged mitochondria release their contents into the cytoplasm and then mediate various signaling pathways. The role of mitochondrial dysfunction in radiation carcinogenesis was discussed, and the recent findings on radiation-induced mitochondrial signaling and radioprotective agents that targeted mitochondria were presented. The analysis of the mitochondrial radiation effect, as hypothesized, is critical in assessing radiation risks to human health.

Melatonin and MitoEbselen-2 Are Radioprotective Agents to Mitochondria

Mitochondria are responsible for controlling cell death during the early stages of radiation exposure, but their perturbations are associated with late effects of radiation-related carcinogenesis. Therefore, it is important to protect mitochondria to mitigate the harmful effects of radiation throughout life. The glutathione peroxidase (GPx) enzyme is essential for the maintenance of mitochondrial-derived reactive oxygen species (ROS) levels. However, radiation inactivates the GPx, resulting in metabolic oxidative stress and prolonged cell injury in irradiated normal human fibroblasts. Here, we used the GPx activator N-acetyl-5-methoxy-tryptamine (melatonin) and a mitochondria-targeted mimic of GPx MitoEbselen-2 to stimulate the GPx. A commercial GPx activity assay kit was used to measure the GPx activity. ROS levels were determined by using some ROS indicators. Protein expression associated with the response of mitochondria to radiation was assessed using immunostaining. Concurrent pre-administration or post-administration of melatonin or MitoEbselen-2 with radiation maintained GPx activity and ROS levels and suppressed mitochondrial radiation responses associated with cellular damage and radiation-related carcinogenesis. In conclusion, melatonin and MitoEbselen-2 prevented radiation-induced mitochondrial injury and metabolic oxidative stress by targeting mitochondria. These drugs have the potential to protect against acute radiation injury and late effects of carcinogenesis in a variety of radiation scenarios assuming pre-administration or post-administration.

Effects of oxygen on the response of mitochondria to X-irradiation and reactive oxygen species-mediated fibroblast activation

PURPOSE

In living organisms, sensitivity to radiation increases in the presence of oxygen (O₂) compared with that under anoxic or hypoxic conditions. Here, we investigated whether O₂ concentration affected the response of mitochondria to X-rays radiation, which is associated with tumor microenvironment formation via fibroblast activation in radiation-related tumors.

MATERIALS AND METHODS

O₂ concentrations were controlled at <5% (internal environmental oxygen condition) or anoxic levels during culture of normal human diploid lung fibroblasts TIG-3 and MRC-5. Protein expression associated with the response of mitochondria to radiation was assessed using immunostaining or western blotting.

RESULTS

Induction of DNA damage (marker: γ-H2A histone family member X) and mitochondrial signaling (AMP-activated protein kinase), suppression of mitochondrial metabolic activity, and generation of reactive oxygen species occurred with radiation in cells cultured under 5% and 20% O₂ conditions. However, reducing O₂ concentration mitigated the effects of radiation on cell growth, mitochondrial damage (parkin), induction of antioxidant responses (nuclear factor E2-related factor 2), and fibroblast activation (α-smooth muscle actin). Radiation did not affect the markers used in this study in the absence of O₂.

CONCLUSION

O₂ concentration affected the response of mitochondria to radiation and reactive oxygen species-mediated fibroblast activation. Higher O₂ concentrations enhanced the effects of radiation on mitochondria in human fibroblasts. In vitro studies may overestimate in vivo radiation effects due to high O₂ concentrations.

Radiation affects glutathione redox reaction by reduced glutathione peroxidase activity in human fibroblasts

The glutathione (GSH) redox control is critical to maintain redox balance in the body's internal environment, and its perturbation leads to a dramatic increase in reactive oxygen species (ROS) levels and oxidative stress which have negative impacts on human health. Although ionizing radiation increases mitochondrial ROS generation, the mechanisms underlying radiation-induced late ROS accumulation are not fully understood. Here we investigated the radiation effect on GSH redox reactions in normal human diploid lung fibroblasts TIG-3 and MRC-5. Superoxide anion probe MitoSOX-red staining and measurement of GSH peroxidase (GPx) activity revealed that high dose single-radiation (SR) exposure (10 Gy) increased mitochondrial ROS generation and overall oxidative stress in parallel with decrease in GSH peroxidase (GPx) activity, while GSH redox control was effective after exposure to moderate doses under standard serum conditions. We used different serum conditions to elucidate the role of serum on GSH redox reaction. Serum starvation, serum deprivation and DNA damage response (DDR) inhibitors-treatment reduced the GPx activity and increased mitochondrial ROS generation regardless of radiation exposure. Fractionated-radiation was used to evaluate the radiation effect on GSH reactions. Repeated fractionated-radiation induced prolonged oxidative stress by down-regulation of GPx activity. In conclusion, radiation affects GSH usage according to radiation dose, irradiation methods and serum concentration. Radiation affected the GPx activity to disrupt fibroblast redox homeostasis.

Advances in the Current Understanding of How Low-Dose Radiation Affects the Cell Cycle

Cells exposed to ionizing radiation undergo a series of complex responses, including DNA damage, reproductive cell death, and altered proliferation states, which are all linked to cell cycle dynamics. For many years, a great deal of research has been conducted on cell cycle checkpoints and their regulators in mammalian cells in response to high-dose exposures to ionizing radiation. However, it is unclear how low-dose ionizing radiation (LDIR) regulates the cell cycle progression. A growing body of evidence demonstrates that LDIR may have profound effects on cellular functions. In this review, we summarize the current understanding of how LDIR (of up to 200 mGy) regulates the cell cycle and cell-cycle-associated proteins in various cellular settings. In light of current findings, we also illustrate the conceptual function and possible dichotomous role of p21^Waf1, a transcriptional target of p53, in response to LDIR.

Low Dose Ionising Radiation-Induced Hormesis: Therapeutic Implications to Human Health

The concept of radiation-induced hormesis, whereby a low dose is beneficial and a high dose is detrimental, has been gaining attention in the fields of molecular biology, environmental toxicology and radiation biology. There is a growing body of literature that recognises the importance of hormetic dose response not only in the radiation field, but also with molecular agents. However, there is continuing debate on the magnitude and mechanism of radiation hormetic dose response, which could make further contributions, as a research tool, to science and perhaps eventually to public health due to potential therapeutic benefits for society. The biological phenomena of low dose ionising radiation (LDIR) includes bystander effects, adaptive response, hypersensitivity, radioresistance and genomic instability. In this review, the beneficial and the detrimental effects of LDIR-induced hormesis are explored, together with an overview of its underlying cellular and molecular mechanisms that may potentially provide an insight to the therapeutic implications to human health in the future.

ATM-Mediated Mitochondrial Radiation Responses of Human Fibroblasts

Ataxia telangiectasia (AT) is characterized by extreme sensitivity to ionizing radiation. The gene mutated in AT, Ataxia Telangiectasia Mutated (ATM), has serine/threonine protein kinase activity and mediates the activation of multiple signal transduction pathways involved in the processing of DNA double-strand breaks. Reactive oxygen species (ROS) created as a byproduct of the mitochondria's oxidative phosphorylation (OXPHOS) has been proposed to be the source of intracellular ROS. Mitochondria are uniquely vulnerable to ROS because they are the sites of ROS generation. ROS-induced mitochondrial mutations lead to impaired mitochondrial respiration and further increase the likelihood of ROS generation, establishing a vicious cycle of further ROS production and mitochondrial damage. AT patients and ATM-deficient mice display intrinsic mitochondrial dysfunction and exhibit constitutive elevations in ROS levels. ATM plays a critical role in maintaining cellular redox homeostasis. However, the precise mechanism of ATM-mediated mitochondrial antioxidants remains unclear. The aim of this review paper is to introduce our current research surrounding the role of ATM on maintaining cellular redox control in human fibroblasts. ATM-mediated signal transduction is important in the mitochondrial radiation response. Perturbation of mitochondrial redox control elevates ROS which are key mediators in the development of cancer by many mechanisms, including ROS-mediated genomic instability, tumor microenvironment formation, and chronic inflammation.

Pleiotropic and Potentially Beneficial Effects of Reactive Oxygen Species on the Intracellular Signaling Pathways in Endothelial Cells

Endothelial cells (ECs) are exposed to molecular dioxygen and its derivative reactive oxygen species (ROS). ROS are now well established as important signaling messengers. Excessive production of ROS, however, results in oxidative stress, a significant contributor to the development of numerous diseases. Here, we analyze the experimental data and theoretical concepts concerning positive pro-survival effects of ROS on signaling pathways in endothelial cells (ECs). Our analysis of the available experimental data suggests possible positive roles of ROS in induction of pro-survival pathways, downstream of the G_i-protein-coupled receptors, which mimics insulin signaling and prevention or improvement of the endothelial dysfunction. It is, however, doubtful, whether ROS can contribute to the stabilization of the endothelial barrier.

The role of mitochondrial oxidative stress and the tumor microenvironment in radiation-related cancer

The health risks associated with low-dose radiation, which are a major concern after the Fukushima Daiichi nuclear power plant accident (the Fukushima accident), have been extensively investigated, and the cancer risks from low-dose radiation exposure (below ~ 100 mSv) are thought to be negligible. According to World Health Organization and the United Nations Scientific Committee on the Effects of Atomic Radiation reports, the level of radiation exposure from the Fukushima accident is limited, estimating no significant increased risk from the accident. Radiation-induced cell injury is mainly caused by oxidative damage to biomolecules, including DNA, lipids and proteins. Radiation stimulates metabolic activation within the mitochondria to provide energy for the DNA damage response. Mitochondrial respiratory chain complexes I and III are the most important intracellular source of reactive oxygen species (ROS) during oxidative phosphorylation in eukaryotic cells. Manganese superoxide dismutase and glutathione are key players in redox control within cells. However, perturbation of the antioxidant response leads to chronic oxidative stress in irradiated cells. Excess ROS of mitochondrial origin is reported in cancer-associated fibroblast and promotes carcinogenesis. The aim of this review paper is to discuss critical roles of mitochondria in radiation-related cancer by introducing our recent studies. In particular, elevated mitochondrial ROS in stromal fibroblasts potentiate transforming growth factor-beta (TGF-β) signaling, which triggers smooth muscle actin (α-SMA) expression to stimulate myofibroblast differentiation. Radiation-induced myofibroblasts promote tumor growth by enhancing angiogenesis. Thus, radiation affects both malignant cancer cells and neighboring stromal cells through secretion of soluble factors.

Mitochondrial reactive oxygen species in physiology and disease

Mitochondrial reactive oxygen species (mROS) are routinely produced at several sites within the organelle. The balance in their formation and elimination is maintained by a complex and robust antioxidant system. mROS may act as second messengers and regulate a number of physiological processes, such as insulin signaling, cell differentiation and proliferation, wound healing, etc. Nevertheless, when a sudden or sustained increase in ROS formation is not efficiently neutralized by the endogenous antioxidant defense system, the detrimental impact of high mROS levels on cell function and viability eventually results in disease development. In this review, we will focus on the dual role of mROS in pathophysiology, emphasizing the physiological role exerted by a regulated mROS production/elimination, and discussing the detrimental effects evoked by an imbalance in mitochondrial redox state. Furthermore, we will touch upon the interplay between mROS and Ca²⁺ homeostasis.

Roles of Fibroblasts in Microenvironment Formation Associated with Radiation-Induced Cancer

In tumor tissues, activated stromal fibroblasts, termed cancer-associated fibroblasts (CAFs), exhibit similar characteristics to myofibroblasts. CAFs promote cancer cell differentiation and invasion by releasing various factors, such as growth factors, chemokines, and matrix-degrading proteases, into neighboring tumor cells. However, the roles of tumor microenvironment in case of radiation-induced carcinogenesis remain poorly understood. We recently revealed that mitochondrial oxidative stress causes tumor microenvironment formation associated with radiation-induced cancer. Repeated low-dose fractionated radiation progressively damages fibroblast mitochondria and elevates mitochondrial reactive oxygen species (ROS) levels. Excessive mitochondrial ROS activate transforming growth factor-beta (TGF-β) signaling, thereby inducing fibroblasts activation and facilitating tumor microenvironment formation. Consequently, radiation affects malignant cancer cells directly and indirectly via molecular alterations in stromal fibroblasts, such as the activation of TGF-β and angiogenic signaling. This review summarizes for the first time the roles of mitochondrial oxidative stress in microenvironment formation associated with radiation-induced cancer. This review may help us understand the risks of exposure to low-dose radiation. The cross talk between cancer cells and stromal fibroblasts contributes to the development and progression of radiation-induced cancer.

Cyclobutane pyrimidine dimers from UVB exposure induce a hypermetabolic state in keratinocytes via mitochondrial oxidative stress

Ultraviolet B radiation (UVB) is an environmental complete carcinogen, which induces and promotes keratinocyte carcinomas, the most common human malignancies. UVB induces the formation of cyclobutane pyrimidine dimers (CPDs). Repairing CPDs through nucleotide excision repair is slow and error-prone in placental mammals. In addition to the mutagenic and malignancy-inducing effects, UVB also elicits poorly understood complex metabolic changes in keratinocytes, possibly through CPDs. To determine the effects of CPDs, CPD-photolyase was overexpressed in keratinocytes using an N1-methyl pseudouridine-containing in vitro-transcribed mRNA. CPD-photolyase, which is normally not present in placental mammals, can efficiently and rapidly repair CPDs to block signaling pathways elicited by CPDs. Keratinocytes surviving UVB irradiation turn hypermetabolic. We show that CPD-evoked mitochondrial reactive oxygen species production, followed by the activation of several energy sensor enzymes, including sirtuins, AMPK, mTORC1, mTORC2, p53, and ATM, is responsible for the compensatory metabolic adaptations in keratinocytes surviving UVB irradiation. Compensatory metabolic changes consist of enhanced glycolytic flux, Szent-Györgyi-Krebs cycle, and terminal oxidation. Furthermore, mitochondrial fusion, mitochondrial biogenesis, and lipophagy characterize compensatory hypermetabolism in UVB-exposed keratinocytes. These properties not only support the survival of keratinocytes, but also contribute to UVB-induced differentiation of keratinocytes. Our results indicate that CPD-dependent signaling acutely maintains skin integrity by supporting cellular energy metabolism.

Mechanism of turnover or persistence of radiation-induced myofibroblast in vitro

We recently made an important discovery that radiation induces myofibroblasts, which play a role in radiation-related carcinogenesis via tumor microenvironment formation. Here, we investigated the threshold dose and the mechanisms of myofibroblast induction to assess adverse radiation effects on normal cells. Single-dose of healthy human fibroblasts in vitro promotes myofibroblast induction at high doses (≥ 5 Gy). In contrast, repeated low dose of fractionated radiation is at least equivalent to high-dose single radiation regarding myofibroblast induction. ROS play a pivotal role in the process of myofibroblast induction in normal tissue injury. Antioxidants, such as epicatechin and ascorbic acid can prevent myofibroblast induction by scavenging ROS. We further investigated the role of DNA damage responses (DDR) on myofibroblast induction. Blocking the DDR using DNA-PK or AKT inhibitors enhanced cellular sensitivity to radiation and facilitated myofibroblast induction, whereas an ATM inhibitor also enhanced radiation sensitivity but abrogated ROS accumulation and myofibroblast induction. In contrast to standard culture conditions, myofibroblasts remained after low or moderate doses of radiation (below 2.5 Gy) under growth-restricted conditions. In conclusion, the recovery of damaged cells from radiation is essential for myofibroblast clearance, which restores stromal cell dormancy and prevents tumor microenvironment formation. However, residual ROS, by way of sustaining myofibroblast presence, can facilitate tumor microenvironment formation. Targeting ROS using antioxidants is effective in the mitigation of radiation-related adverse effects, such as growth retardation and myofibroblast induction, and helps protect normal tissues.

Targeted Nuclear Irradiation with an X-Ray Microbeam Enhances Total JC-1 Fluorescence from Mitochondria

Several studies have demonstrated that mitochondria are critically involved in the pleiotropic manifestation of radiation effects. While conventional whole-cell irradiation compromises the function of mitochondria, the effects of subcellular targeted radiation are not yet fully understood. In this study, normal human diploid cells with cell-cycle indicators were irradiated using a synchrotron X-ray microbeam, and mitochondrial membrane potential was quantified by JC-1 over the 72-h period postirradiation. Cytoplasmic irradiation was observed to temporarily enlarge the mitochondrial area with high membrane potential, while the total mitochondrial area did not change significantly. Unexpectedly, cell-nucleus irradiation promoted a similar increase not only in the mitochondrial areas with high membrane potential, but also in those with low membrane potential, which gave rise to the apparent increase in the total mitochondrial area. Augmentation of the mitochondrial area with low membrane potential was predominantly observed among G1 cells, suggesting that nucleus irradiation during the G1 phase regulated the mitochondrial dynamics of the cytoplasm, presumably through DNA damage in the nucleus.

Riding the tiger - physiological and pathological effects of superoxide and hydrogen peroxide generated in the mitochondrial matrix

Elevated mitochondrial matrix superoxide and/or hydrogen peroxide concentrations drive a wide range of physiological responses and pathologies. Concentrations of superoxide and hydrogen peroxide in the mitochondrial matrix are set mainly by rates of production, the activities of superoxide dismutase-2 (SOD2) and peroxiredoxin-3 (PRDX3), and by diffusion of hydrogen peroxide to the cytosol. These considerations can be used to generate criteria for assessing whether changes in matrix superoxide or hydrogen peroxide are both necessary and sufficient to drive redox signaling and pathology: is a phenotype affected by suppressing superoxide and hydrogen peroxide production; by manipulating the levels of SOD2, PRDX3 or mitochondria-targeted catalase; and by adding mitochondria-targeted SOD/catalase mimetics or mitochondria-targeted antioxidants? Is the pathology associated with variants in SOD2 and PRDX3 genes? Filtering the large literature on mitochondrial redox signaling using these criteria highlights considerable evidence that mitochondrial superoxide and hydrogen peroxide drive physiological responses involved in cellular stress management, including apoptosis, autophagy, propagation of endoplasmic reticulum stress, cellular senescence, HIF1α signaling, and immune responses. They also affect cell proliferation, migration, differentiation, and the cell cycle. Filtering the huge literature on pathologies highlights strong experimental evidence that 30-40 pathologies may be driven by mitochondrial matrix superoxide or hydrogen peroxide. These can be grouped into overlapping and interacting categories: metabolic, cardiovascular, inflammatory, and neurological diseases; cancer; ischemia/reperfusion injury; aging and its diseases; external insults, and genetic diseases. Understanding the involvement of mitochondrial matrix superoxide and hydrogen peroxide concentrations in these diseases can facilitate the rational development of appropriate therapies.

Induction of oxidative stress biomarkers following whole-body irradiation in mice

Dose assessment is an important issue for radiation emergency medicine to determine appropriate clinical treatment. Hematopoietic tissues are extremely vulnerable to radiation exposure. A decrease in blood cell count following radiation exposure is the first quantitative bio-indicator using hematological techniques. We further examined induction of oxidative stress biomarkers in residual lymphocytes to identify new biomarkers for dosimetry. In vivo whole-body radiation to mice exposed to 5 Gy significantly induces DNA double-strand breaks, which were visualized by γ-H2AX in mouse blood cells. Mouse blood smears and peripheral blood mononuclear cells (PBMC) isolated from irradiated mice were used for immunostaining for oxidative biomarkers, parkin or Nrf2. Parkin is the E3 ubiquitin ligase, which is normally localized in the cytoplasm, is relocated to abnormal mitochondria with low membrane potential (ΔΨm), where it promotes clearance via mitophagy. Nrf2 transcription factor controls the major cellular antioxidant responses. Both markers of oxidative stress were more sensitive and persistent over time than nuclear DNA damage. In conclusion, parkin and Nrf2 are potential biomarkers for use in radiation dosimetry. Identification of several biological markers which show different kinetics for radiation response is essential for radiation dosimetry that allows the assessment of radiation injury and efficacy of clinical treatment in emergency radiation incidents. Radiation-induced oxidative damage is useful not only for radiation dose assessment but also for evaluation of radiation risks on humans.

Roles of Reactive Oxygen Species in Biological Behaviors of Prostate Cancer

Prostate cancer (PCa), known as a heterogenous disease, has a high incidence and mortality rate around the world and seriously threatens public health. As an inevitable by-product of cellular metabolism, reactive oxygen species (ROS) exhibit beneficial effects by regulating signaling cascades and homeostasis. More and more evidence highlights that PCa is closely associated with age, and high levels of ROS are driven through activation of several signaling pathways with age, which facilitate the initiation, development, and progression of PCa. Nevertheless, excessive amounts of ROS result in harmful effects, such as genotoxicity and cell death. On the other hand, PCa cells adaptively upregulate antioxidant genes to detoxify from ROS, suggesting that a subtle balance of intracellular ROS levels is required for cancer cell functions. The current review discusses the generation and biological roles of ROS in PCa and provides new strategies based on the regulation of ROS for the treatment of PCa.

Interplay between BMPs and Reactive Oxygen Species in Cell Signaling and Pathology

The integration of cell extrinsic and intrinsic signals is required to maintain appropriate cell physiology and homeostasis. Bone morphogenetic proteins (BMPs) are cytokines that belong to the transforming growth factor-β (TGF-β) superfamily, which play a key role in embryogenesis, organogenesis and regulation of whole-body homeostasis. BMPs interact with membrane receptors that transduce information to the nucleus through SMAD-dependent and independent pathways, including PI3K-AKT and MAPKs. Reactive oxygen species (ROS) are intracellular molecules derived from the partial reduction of oxygen. ROS are highly reactive and govern cellular processes by their capacity to regulate signaling pathways (e.g., NF-κB, MAPKs, KEAP1-NRF2 and PI3K-AKT). Emerging evidence indicates that BMPs and ROS interplay in a number of ways. BMPs stimulate ROS production by inducing NOX expression, while ROS regulate the expression of several BMPs. Moreover, BMPs and ROS influence common signaling pathways, including PI3K/AKT and MAPK. Additionally, dysregulation of BMPs and ROS occurs in several pathologies, including vascular and musculoskeletal diseases, obesity, diabetes and kidney injury. Here, we review the current knowledge on the integration between BMP and ROS signals and its potential applications in the development of new therapeutic strategies.

Interplay Between Mitochondrial Peroxiredoxins and ROS in Cancer Development and Progression

Mitochondria are multifunctional cellular organelles that are major producers of reactive oxygen species (ROS) in eukaryotes; to maintain the redox balance, they are supplemented with different ROS scavengers, including mitochondrial peroxiredoxins (Prdxs). Mitochondrial Prdxs have physiological and pathological significance and are associated with the initiation and progression of various cancer types. In this review, we have focused on signaling involving ROS and mitochondrial Prdxs that is associated with cancer development and progression. An upregulated expression of Prdx3 and Prdx5 has been reported in different cancer types, such as breast, ovarian, endometrial, and lung cancers, as well as in Hodgkin's lymphoma and hepatocellular carcinoma. The expression of Prdx3 and Prdx5 in different types of malignancies involves their association with different factors, such as transcription factors, micro RNAs, tumor suppressors, response elements, and oncogenic genes. The microenvironment of mitochondrial Prdxs plays an important role in cancer development, as cancerous cells are equipped with a high level of antioxidants to overcome excessive ROS production. However, an increased production of Prdx3 and Prdx5 is associated with the development of chemoresistance in certain types of cancers and it leads to further complications in cancer treatment. Understanding the interplay between mitochondrial Prdxs and ROS in carcinogenesis can be useful in the development of anticancer drugs with better proficiency and decreased resistance. However, more targeted studies are required for exploring the tumor microenvironment in association with mitochondrial Prdxs to improve the existing cancer therapies and drug development.

Effects of PP2A/Nrf2 on experimental diabetes mellitus-related cardiomyopathy by regulation of autophagy and apoptosis through ROS dependent pathway

Diabetes mellitus-related cardiomyopathy (DMCMP) has been defined as ventricular dysfunction that occurs in diabetic patients independent of a recognized cause such as coronary artery disease or hypertension. Mechanisms underlying DMCMP have not been fully elucidated. In this study, the roles of protein phosphatase 2A/nuclear factor NF-E2-related factor 2 (PP2A/Nrf2) in experimental DMCMP induced by high glucose were studied in vitro and in vivo. The results showed that high glucose could induce experimental DMCMP and increase ROS generation, increase the expression and nuclear translocation of Nrf2, down-regulate the expression of PI3K/Akt/mTOR and up-regulate the expression of ERK, and activate the autophagy of cardiomyocytes. The activity or expression of PP2A in DMCMP increased. PP2A could up-regulate the expression of Nrf2 and promote cardiomyocytes autophagy and apoptosis. Inhibition of PP2A could reduce the expression of Nrf2 and inhibit the autophagy and apoptosis of cardiomyocytes. The results suggested that hyperglycemic-induced experimental DMCMP may be related to up-regulating the expression of Nrf2 through PP2A/Nrf2 pathway. These results will be helpful to elucidate the pathogenesis and mechanism of DMCMP and find targets for the development of new drugs to prevent or treat DMCMP.

Senescent Hepatocytes in Decompensated Liver Show Reduced UPRMT and Its Key Player, CLPP, Attenuates Senescence In Vitro

BACKGROUND AND AIMS

Non-dividing hepatocytes in end-stage liver disease indicates permanent growth arrest similar to senescence. Identifying senescence in vivo is often challenging and mechanisms inhibiting senescence are poorly understood. In lower organisms mitochondrial unfolded protein response (UPR^MT) helps in increasing longevity; however, its role in senescence and liver disease is poorly understood. Aim of this study was to identify hepatocyte senescence and the role of UPR^MT in cryptogenic cirrhosis.

METHODS

Doxorubicin was used to induce senescence in non-neoplastic hepatocytes (PH5CH8) and hepatoma cells (HepG2 and Huh7). Senescence-associated markers and unfolded protein response was evaluated by fluorescence microscopy, immunoblotting and gene expression. Explants/biopsies from normal, fibrosis, compensated and decompensated cirrhosis without any known etiology were examined for presence of senescence and UPR^MT by immunohistochemistry and gene expression.

RESULTS

Accumulation of senescent hepatocytes in cryptogenic cirrhosis was associated with reduced proliferation, increased expression of γH2AX and p21, together with loss of LaminB1. Dysfunctional mitochondria and compromised UPR^MT were key features of senescent hepatocytes both in vitro and also in decompensated cirrhosis. Intriguingly, compensated cirrhotic liver mounted strong UPR^MT, with high levels of mitochondrial protease, CLPP. Overexpression of CLPP inhibited senescence in vitro, by reducing mitochondrial ROS and altering oxygen consumption.

CONCLUSIONS

Our results implicate a role of hepatocyte senescence in cryptogenic cirrhosis together with a crucial role of UPR^MT in preventing hepatocyte senescence. A compromised UPR^MT may shift the fate of cirrhotic liver toward decompensation by exaggerating hepatocyte senescence. Restoring CLPP levels at least in cell culture appears as a promising strategy in mitohormesis, thereby, preventing senescence and possibly improving hepatocyte function.

Epicatechin as a promising agent to countermeasure radiation exposure by mitigating mitochondrial damage in human fibroblasts and mouse hematopoietic cells

Accidental radiation exposure that is due to a nuclear accident or terrorism using radioactive materials has severe detrimental effects on human health, and it can manifest as acute radiation syndrome depending on the dose and distribution of the radiation. Therefore, the development of radiation countermeasure agents is urgently needed to protect humans against radiation injury. Besides nuclear DNA, the mitochondria are important targets of ionizing radiation (IR) because these organelles generate reactive oxygen species (ROS). Recently, we revealed that mitochondrial ROS-activated cell signaling is associated with IR-induced tumor formation. Here, we investigated the effectiveness of ascorbic acid and epicatechin (EC) in scavenging ROS as radiation countermeasure agents by using human cells and mouse. Preradiation and postradiation treatments with EC mitigate ROS-mediated mitochondrial damage, IR-induced oxidative stress responses including reduction of superoxide dismutase activity, and elevated nuclear factor erythroid 2-related factor 2 expression, and they improve human fibroblast survival. As well as in vitro, EC mitigated ROS-mediated mitochondrial damage after exposure to IR in vivo in mouse platelets. Furthermore, oral administration of EC significantly enhanced the recovery of mouse hematopoietic cells from radiation injury in vivo. In summary, EC is a potentially viable countermeasure agent that is immediately effective against accidental IR exposure by targeting mitochondria-mediated oxidative stress.-Shimura, T., Koyama, M., Aono, D., Kunugita, N. Epicatechin as a promising agent to countermeasure radiation exposure by mitigating mitochondrial damage in human fibroblasts and mouse hematopoietic cells.

Regulation of the phosphoprotein phosphatase 2A system and its modulation during oxidative stress: A potential therapeutic target?

Phosphoprotein phosphatases are of growing interest in the pathophysiology of many diseases and are often the neglected partner of protein kinases. One family member, PP2A, accounts for dephosphorylation of ~55-70% of all serine/threonine phosphosites. Interestingly, dysregulation of kinase signalling is a hallmark of many diseases in which an increase in oxidative stress is also noted. With this in mind, we assess the evidence to support oxidative stress-mediated regulation of the PP2A system In this article, we first present an overview of the PP2A system before providing an analysis of the regulation of PP2A by endogenous inhibitors, post translational modification, and miRNA. Next, a detailed critique of data implicating reactive oxygen species, ischaemia, ischaemia-reperfusion, and hypoxia in regulating the PP2A holoenzyme and associated regulators is presented. Finally, the pharmacological targeting of PP2A, its endogenous inhibitors, and enzymes responsible for its post-translational modification are covered. There is extensive evidence that oxidative stress modulates multiple components of the PP2A system, however, most of the data pertains to the catalytic subunit of PP2A. Irrespective of the underlying aetiology, free radical-mediated attenuation of PP2A activity is an emerging theme. However, in many instances, a dichotomy exists, which requires clarification and mechanistic insight. Nevertheless, this raises the possibility that pharmacological activation of PP2A, either through small molecule activators of PP2A or CIP2A/SET antagonists may be beneficial in modulating the cellular response to oxidative stress. A better understanding of which, will have wide ranging implications for cancer, heart disease and inflammatory conditions.

Re-evaluation of the linear no-threshold (LNT) model using new paradigms and modern molecular studies

The linear no-threshold (LNT) model is currently used to estimate low dose radiation (LDR) induced health risks. This model lacks safety thresholds and postulates that health risks caused by ionizing radiation is directly proportional to dose. Therefore even the smallest radiation dose has the potential to cause an increase in cancer risk. Advances in LDR biology and cell molecular techniques demonstrate that the LNT model does not appropriately reflect the biology or the health effects at the low dose range. The main pitfall of the LNT model is due to the extrapolation of mutation and DNA damage studies that were conducted at high radiation doses delivered at a high dose-rate. These studies formed the basis of several outdated paradigms that are either incorrect or do not hold for LDR doses. Thus, the goal of this review is to summarize the modern cellular and molecular literature in LDR biology and provide new paradigms that better represent the biological effects in the low dose range. We demonstrate that LDR activates a variety of cellular defense mechanisms including DNA repair systems, programmed cell death (apoptosis), cell cycle arrest, senescence, adaptive memory, bystander effects, epigenetics, immune stimulation, and tumor suppression. The evidence presented in this review reveals that there are minimal health risks (cancer) with LDR exposure, and that a dose higher than some threshold value is necessary to achieve the harmful effects classically observed with high doses of radiation. Knowledge gained from this review can help the radiation protection community in making informed decisions regarding radiation policy and limits.

Use of DNA-generated gold nanoparticles to radiosensitize and eradicate radioresistant glioma stem cells

The surface reactivity of gold nanoparticles (AuNPs) is receiving attention as a radiosensitizer of cancer cells for radiation therapy and/or as a drug carrier to target cells. This study demonstrates the potential of DNA-AuNPs (prepared by mixing calf thymus DNA with HAuCl₄ solution) as a radiosensitizer of human glioma cells that have cancer stem cell (CSC)-like properties, to reduce their survival. CSC-like U251MG-P1 cells and their parental glioblastoma U251MG cells are treated with a prepared DNA-AuNP colloid. The radiosensitivity of the resultant AuNP-associated cells are significantly enhanced. To reveal the mechanism by which survival is reduced, the generation of reactive oxygen species (ROS), apoptosis induction, or DNA damage in the cells is assayed using the fluorescent dye DCFDA, annexin V-FITC/PI, and foci formation of γ-H2AX, respectively. X-ray irradiation with administration of AuNPs overcomes the radioresistance of U251MG-P1 cells. It does not induce ROS generation or apoptosis in the cells but enhances the number of abnormal nuclei with abundant γ-H2AX foci, which is judged as cell death by mitotic catastrophe. The AuNP association with the cells effectively induces mitotic catastrophe in x-ray-irradiated CSC-like cells, implicating that DNA-AuNPs might be a promising tool to develop an efficient radiosensitizer against CSC.

Enhanced combination cancer therapy using lipid-calcium carbonate/phosphate nanoparticles as a targeted delivery platform

Aim: Melanoma, the most life-threatening skin cancer, requires more effective therapies. Methodology: A new folic acid (FA) receptor-targeted lipid-coated calcium carbonate/phosphate (LCCP) nanoparticle was synthesized, incorporating two often-used therapeutics, cell death siRNA and α-tocopheryl succinate. Results: The nanoparticles were spherical, with an average size of 40 nm. The nanoparticles exhibited a high gene/drug loading efficiency (60%), with folic acid-enhanced cellular uptake. The nanoparticles with both therapeutics enhanced inhibition of B16F0 melanoma cell growth, showing a moderate synergistic effect. The mechanism of the inhibition is associated with induction of cell apoptosis and cell cycle arrest at G1 phase. Conclusion: Our data indicate that lipid-coated calcium carbonate/phosphate nanoparticles are a potential platform for targeted therapy for melanoma.

Investigation into Cellular Glycolysis for the Mechanism Study of Energy Metabolism Disorder Triggered by Lipopolysaccharide

Lipopolysaccharide (LPS) is the main virulence factor of Gram-negative bacteria, which can incite inflammation in tissues by inducing cells to secrete a variety of proinflammatory mediators, including cytokines, chemokines, interleukins, and prostaglandins. Herein, we chose LPS as an inducer to establish an inflammatory model of HeLa cells, and explored the effects of LPS on energy metabolism. We treated HeLa cells with different concentrations (0, 0.4, 1.0, 2.0, 4.0, and 6.0 μg/mL) of LPS for 24 h, and explored its effects on intercellular adenosine triphosphate (ATP) levels, intercellular nitrous oxide (NO) content, mitochondrial functions, and enzyme activities related to energy metabolism. Furthermore, we used metabonomics to study the metabolites that participated in energy metabolism. We found a positive correlation between LPS concentrations and intracellular ATP levels. In addition, LPS increased intracellular NO production, altered mitochondrial functions, strengthened glycolytic enzyme activities, and changed metabolites related to energy metabolism. Hence, in this study, we showed that LPS can strengthen energy metabolism by enhancing glycolysis, which could be used as an early diagnostic biomarker or a novel therapeutic target for inflammation-associated cancers.

Radiation-Induced Myofibroblasts Promote Tumor Growth via Mitochondrial ROS-Activated TGFβ Signaling

Fibroblasts are a key stromal cell in the tumor microenvironment (TME) and promote tumor growth via release of various growth factors. Stromal fibroblasts in cancer, called cancer-associated fibroblasts (CAF), are related to myofibroblasts, an activated form of fibroblast. While investigating the role of stroma fibroblasts on radiation-related carcinogenesis, it was observed following long-term fractionated radiation (FR) that the morphology of human diploid fibroblasts changed from smaller spindle shapes to larger flat shapes. These cells expressed smooth muscle actin (α-SMA) and platelet-derived growth factor receptors, markers of myofibroblasts and CAFs, respectively. Long-term FR induces progressive damage to the fibroblast nucleus and mitochondria via increases in mitochondrial reactive oxygen species (ROS) levels. Here, it is demonstrated that long-term FR-induced α-SMA-positive cells have decreased mitochondrial membrane potential and activated oxidative stress responses. Antioxidant N-acetyl cysteine suppressed radiation-induced mitochondrial damage and generation of myofibroblasts. These results indicate that mitochondrial ROS are associated with the acquisition of myofibroblasts after long-term FR. Mechanistically, mitochondrial ROS activated TGFβ signaling which in turn mediated the expression of α-SMA in radiation-induced myofibroblasts. Finally, in vivo tumor growth analysis in a human tumor xenograft model system revealed that long-term FR-induced myofibroblasts promote tumor growth by enhancing angiogenesis.Implications: Radiation affects malignant cancer cells directly and indirectly via molecular alterations in stromal fibroblasts such as activation of TGFβ and angiogenic signaling pathways. Mol Cancer Res; 16(11); 1676-86. ©2018 AACR.

Ionizing Radiation-Induced Cellular Senescence in Normal, Non-transformed Cells and the Involved DNA Damage Response: A Mini Review

Cellular senescence is identified by a living cell in irreversible and persistent cell cycle arrest in response to various cellular stresses. Senescent cells secrete senescence-associated secretory phenotype factors that can amplify cellular senescence and alter the microenvironments. Radiotherapy, via ionizing radiation, serves as an effective treatment for local tumor control with side effects on normal cells, which can induce inflammation and fibrosis in irradiated and nearby regions. Research has revealed that senescent phenotype is observable in irradiated organs. This process starts with DNA damage mediated by radiation, after which a G2 arrest occurs in virtually all eukaryotic cells and a mitotic bypass is possibly necessary to ultimately establish cellular senescence. Within this complex DNA damage response signaling network, ataxia telangiectasia-mutated protein, p53, and p21 stand out as the crucial mediators. Senolytic agents, a class of small molecules that can selectively kill senescent cells, hold great potential to substantially reduce the side effects caused by radiotherapy while reasonably steer clear of carcinogenesis.

Decreased MiR-17 in glioma cells increased cell viability and migration by increasing the expression of Cyclin D1, p-Akt and Akt

BACKGROUND

The activating mutations of micro RNA (miR)-17 have been revealed in tumors such as human non-Hodgkin's lymphoma and T cell leukemia. However, it is unclear about the role of miR-17 in glioma cells. The current study aimed to investigate effects of miR-17 mimics or inhibitor on the viability and migration of rat glioma C6 cells, and explore possible mechanisms.

METHODS

The expression of miR-17 in rat glioma C6 cells and normal brain tissue was detected by quantitative PCR. Protein expression of Cyclin D1 in rat glioma C6 cells and normal brain tissue was measured by Western Blot. Glioma C6 cells were transfected with MiR-17 mimics or inhibitor. Cells that were not transfected (Lipofectamine only) and cells that were transfected with nonsense RNA negative control served as control. MTT assay was utilized to detect cell viability, and cell wound scratch assay was utilized to examine the migration index. In addition, protein expression of Cyclin D1, p-Akt and Akt in MiR-17 mimics or inhibitor-transfected glioma C6 cells was detected by Western Blot. This study had been approved by the Medical Ethics Committee of the First Affiliated Hospital of Soochow University. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.

RESULTS

The expression of miR-17 was significantly lower, whereas the expression of Cyclin D1 was significantly higher in glioma C6 cells compared to normal brain tissue. MiR-17 mimics decreased the viability and migration of glioma C6 cells markedly at 48 h. In addition, MiR-17 inhibitor increased the viability and migration of glioma C6 cells at 24 and 48 h. The protein expression of Cyclin D1, p-Akt and Akt in glioma C6 cells decreased after transfection with miR-17 mimics for 72 h, and increased after transfection with miR-17 inhibitor for 72 h.

CONCLUSIONS

The reduced miR-17 levels in glioma cells increased cell viability and migration, which correlates with increased expression of Cyclin D1, p-Akt and Akt.

ATM-mediated mitochondrial damage response triggered by nuclear DNA damage in normal human lung fibroblasts

Ionizing radiation (IR) elevates mitochondrial oxidative phosphorylation (OXPHOS) in response to the energy requirement for DNA damage responses. Reactive oxygen species (ROS) released during mitochondrial OXPHOS may cause oxidative damage to mitochondria in irradiated cells. In this paper, we investigated the association between nuclear DNA damage and mitochondrial damage following IR in normal human lung fibroblasts. In contrast to low-doses of acute single radiation, continuous exposure of chronic radiation or long-term exposure of fractionated radiation (FR) induced persistent Rad51 and γ-H2AX foci at least 24 hours after IR in irradiated cells. Additionally, long-term FR increased mitochondrial ROS accompanied with enhanced mitochondrial membrane potential (ΔΨm) and mitochondrial complex IV (cytochrome c oxidase) activity. Mitochondrial ROS released from the respiratory chain complex I caused oxidative damage to mitochondria. Inhibition of ATM kinase or ATM loss eliminated nuclear DNA damage recognition and mitochondrial radiation responses. Consequently, nuclear DNA damage activates ATM which in turn increases ROS level and subsequently induces mitochondrial damage in irradiated cells. In conclusion, we demonstrated that ATM is essential in the mitochondrial radiation responses in irradiated cells. We further demonstrated that ATM is involved in signal transduction from nucleus to the mitochondria in response to IR.

Correlation between Oxidative Stress, Nutrition, and Cancer Initiation

Inadequate or excessive nutrient consumption leads to oxidative stress, which may disrupt oxidative homeostasis, activate a cascade of molecular pathways, and alter the metabolic status of various tissues. Several foods and consumption patterns have been associated with various cancers and approximately 30-35% of the cancer cases are correlated with overnutrition or malnutrition. However, several contradictory studies are available regarding the association between diet and cancer risk, which remains to be elucidated. Concurrently, oxidative stress is a crucial factor for cancer progression and therapy. Nutritional oxidative stress may be induced by an imbalance between antioxidant defense and pro-oxidant load due to inadequate or excess nutrient supply. Oxidative stress is a physiological state where high levels of reactive oxygen species (ROS) and free radicals are generated. Several signaling pathways associated with carcinogenesis can additionally control ROS generation and regulate ROS downstream mechanisms, which could have potential implications in anticancer research. Cancer initiation may be modulated by the nutrition-mediated elevation in ROS levels, which can stimulate cancer initiation by triggering DNA mutations, damage, and pro-oncogenic signaling. Therefore, in this review, we have provided an overview of the relationship between nutrition, oxidative stress, and cancer initiation, and evaluated the impact of nutrient-mediated regulation of antioxidant capability against cancer therapy.

A comparison of radiation-induced mitochondrial damage between neural progenitor stem cells and differentiated cells

Mitochondria play a key role in maintaining cellular homeostasis during stress responses, and mitochondrial dysfunction contributes to carcinogenesis, aging, and neurologic disease. We here investigated ionizing radiation (IR)-induced mitochondrial damage in human neural progenitor stem cells (NSCs), their differentiated counterparts and human normal fibroblasts. Long-term fractionated radiation (FR) with low doses of X-rays for 31 d enhanced mitochondrial activity as evident by elevated mitochondrial membrane potential (ΔΨm) and mitochondrial complex IV (cytochrome c oxidase) activity to fill the energy demands for the chronic DNA damage response in differentiated cells. Subsequent reduction of the antioxidant glutathione via continuous activation of mitochondrial oxidative phosphorylation caused oxidative stress and genomic instability in differentiated cells exposed to long-term FR. In contrast, long-term FR had no effect on the mitochondrial activity in NSCs. This cell type showed efficient DNA repair, no mitochondrial damage, and resistance to long-term FR. After high doses of acute single radiation (SR) (> 5 Gy), cell cycle arrest at the G2 phase was observed in NSCs and human fibroblasts. Under this condition, increase in mitochondria mass, mitochondrial DNA, and intracellular reactive oxygen species (ROS) levels were observed in the absence of enhanced mitochondrial activity. Consequently, cellular senescence was induced by high doses of SR in differentiated cells. In conclusion, we demonstrated that mitochondrial radiation responses differ according to the extent of DNA damage, duration of radiation exposure, and cell differentiation.

Soluble Egg Antigens of Schistosoma japonicum Induce Senescence of Activated Hepatic Stellate Cells by Activation of the FoxO3a/SKP2/P27 Pathway

BACKGROUND

Liver fibrosis was viewed as a reversible process. The activation of hepatic stellate cells (HSCs) is a key event in the process of liver fibrosis. The induction of senescence of HSCs would accelerate the clearance of the activated HSCs. Previously, we demonstrated that soluble egg antigens (SEA) of Schistosoma japonicum promoted the senescence of HSCs via STAT3/P53/P21 pathway. In this paper, our study was aimed to explore whether there are other signaling pathways in the process of SEA-induced HSCs aging and the underlying effect of SKP2/P27 signal on senescent HSCs.

METHODOLOGY/PRINCIPAL FINDINGS

Human hepatic stellate cell line, LX-2 cells, were cultured and stimulated with SEA. Western blot and cellular immunofluorescence analysis were performed to determine the expression of senescence-associated protein, such as P27, SKP2 and FoxO3a. Besides, RNA interfering was applied to knockdown the expression of related protein. The senescence of HSCs was determined by senescence-associated β-gal staining. We found that SEA increased the expression of P27 protein, whereas it inhibited the expression of SKP2 and FoxO3a. Knockdown of P27 as well as overexpression of SKP2 both suppressed the SEA-induced senescence of HSCs. In addition, the nuclear translocation of FoxO3a from the nucleus to the cytoplasm was induced by SEA stimulation.

CONCLUSIONS/SIGNIFICANCE

The present study demonstrates that SEA promotes HSCs senescence through the FoxO3a/SKP2/P27 pathway.

Targeting the AKT/cyclin D1 pathway to overcome intrinsic and acquired radioresistance of tumors for effective radiotherapy

PURPOSE

Radiotherapy (RT) is a powerful tool in the treatment of cancer, having the advantage of preserving normal tissues. Clinical outcomes of RT are significantly improved by technological advances, enabling increased radiation doses directed very specifically to a tumor. However, tumor radioresistance remains a major impediment to effective RT. We have shown that human tumor cells surviving after repeated exposure to fractionated radiation (FR) of X-rays for 1 month have acquired radioresistance through constitutive activation of AKT and downstream cyclin D1 nuclear retention. Tumor radioresistance is also proposed to be an intrinsic characteristic of cancer stem cells (CSC), whose efficient DNA repair is thought to confer this phenotype. We have isolated radioresistant CD133-positive cells following exposure to long-term FR. These cells exhibited the CSC phenotype with activation of the AKT/cyclin D1 pathway. In this review, I summarize our current understanding of the molecular mechanisms underlying tumor radioresistance and propose a strategy for overcoming radioresistance by targeting the AKT/cyclin D1 pathway.

CONCLUSION

Two different mechanisms: acquired radioresistance of surviving tumor cells after RT and intrinsic radioresistance of CSC are associated with tumor radioresistance. Inhibition of the AKT pathway results in radiosensitization of both types of tumor radioresistance.

Crosstalk between autophagy and intracellular radiation response (Review)

Autophagy induced by radiation is critical to cell fate decision. Evidence now sheds light on the importance of autophagy induced by cancer radiotherapy. Traditional view considers radiation can directly or indirectly damage DNA which can activate DNA damage the repair signaling pathway, a large number of proteins participating in DNA damage repair signaling pathway such as p53, ATM, PARP1, FOXO3a, mTOR and SIRT1 involved in autophagy regulation. However, emerging recent evidence suggests radiation can also cause injury to extranuclear targets such as plasma membrane, mitochondria and endoplasmic reticulum (ER) and induce accumulation of ceramide, ROS, and Ca2+ concentration which activate many signaling pathways to modulate autophagy. Herein we review the role of autophagy in radiation therapy and the potent intracellular autophagic triggers induced by radiation. We aim to provide a more theoretical basis of radiation-induced autophagy, and provide novel targets for developing cytotoxic drugs to increase radiosensitivity.

Integration of superoxide formation and cristae morphology for mitochondrial redox signaling

The mitochondrial network provides the central cell's energetic and regulatory unit, which besides ATP and metabolite production participates in cellular signaling through regulated reactive oxygen species (ROS) production and various protein/ion fluxes. The inner membrane forms extensive folds, called cristae, i.e. cavities enfolded from and situated perpendicularly to its inner boundary membrane portion, which encompasses an inner cylinder within the outer membrane tubule. Mitochondrial cristae ultramorphology reflects various metabolic, physiological or pathological states. Since the mitochondrion is typically a predominant superoxide source and generated ROS also serve for the creation of information redox signals, we review known relationships between ROS generation within the respiratory chain complexes of cristae and cristae morphology. Notably, it is emphasized that cristae shape is governed by ATP-synthase dimers, MICOS complexes, OPA1 isoforms and the umbrella of their regulation, and also dependent on local protonmotive force (electrical potential component) in cristae. Cristae are also affected by redox-sensitive kinases/phosphatases or p66SHC. ATP-synthase dimers decrease in the inflated intracristal space, diminishing pH and hypothetically having minimal superoxide formation. Matrix-released signaling superoxide/H₂O₂ is predominantly integrated along mitochondrial tubules, whereas the diffusion of intracristal signaling ROS species is controlled by crista junctions, the widening of which enables specific retrograde redox signaling such as during hypoxic cell adaptation. Other physiological cases of H₂O₂ release from the mitochondrion include the modulation of insulin release in pancreatic β-cells, enhancement of insulin signaling in peripheral tissues, signaling by T-cell receptors, retrograde signaling during the cell cycle and cell differentiation, specifically that of adipocytes.

Mitochondrial reactive oxygen species-mediated genomic instability in low-dose irradiated human cells through nuclear retention of cyclin D1

Mitochondria are associated with various radiation responses, including adaptive responses, mitophagy, the bystander effect, genomic instability, and apoptosis. We recently identified a unique radiation response in the mitochondria of human cells exposed to low-dose long-term fractionated radiation (FR). Such repeated radiation exposure inflicts chronic oxidative stresses on irradiated cells via the continuous release of mitochondrial reactive oxygen species (ROS) and decrease in cellular levels of the antioxidant glutathione. ROS-induced oxidative mitochondrial DNA (mtDNA) damage generates mutations upon DNA replication. Therefore, mtDNA mutation and dysfunction can be used as markers to assess the effects of low-dose radiation. In this study, we present an overview of the link between mitochondrial ROS and cell cycle perturbation associated with the genomic instability of low-dose irradiated cells. Excess mitochondrial ROS perturb AKT/cyclin D1 cell cycle signaling via oxidative inactivation of protein phosphatase 2A after low-dose long-term FR. The resulting abnormal nuclear accumulation of cyclin D1 induces genomic instability in low-dose irradiated cells.

Severe mitochondrial damage associated with low-dose radiation sensitivity in ATM- and NBS1-deficient cells

Low-dose radiation risks remain unclear owing to a lack of sufficient studies. We previously reported that low-dose, long-term fractionated radiation (FR) with 0.01 or 0.05 Gy/fraction for 31 d inflicts oxidative stress in human fibroblasts due to excess levels of mitochondrial reactive oxygen species (ROS). To identify the small effects of low-dose radiation, we investigated how mitochondria respond to low-dose radiation in radiosensitive human ataxia telangiectasia mutated (ATM)- and Nijmegen breakage syndrome (NBS)1-deficient cell lines compared with corresponding cell lines expressing ATM and NBS1. Consistent with previous results in normal fibroblasts, low-dose, long-term FR increased mitochondrial mass and caused accumulation of mitochondrial ROS in ATM- and NBS1-complemented cell lines. Excess mitochondrial ROS resulted in mitochondrial damage that was in turn recognized by Parkin, leading to mitochondrial autophagy (mitophagy). In contrast, ATM- and NBS1-deficient cells showed defective induction of mitophagy after low-dose, long-term FR, leading to accumulation of abnormal mitochondria; this was determined by mitochondrial fragmentation and decreased mitochondrial membrane potential. Consequently, apoptosis was induced in ATM- and NBS1-deficient cells after low-dose, long-term FR. Antioxidant N-acetyl-L-cysteine was effective as a radioprotective agent against mitochondrial damage induced by low-dose, long-term FR among all cell lines, including radiosensitive cell lines. In conclusion, we demonstrated that mitochondria are target organelles of low-dose radiation. Mitochondrial response influences radiation sensitivity in human cells. Our findings provide new insights into cancer risk estimation associated with low-dose radiation exposure.