Targeted cytoplasmic irradiation and autophagy

Suppression of NLRP3 inflammasome activation by astragaloside IV via promotion of mitophagy to ameliorate radiation-induced renal injury in mice

Background

Irradiation (IR) promotes inflammation and apoptosis by inducing oxidative stress and/or mitochondrial dysfunction (MD). The kidneys are rich in mitochondria, and mitophagy maintains normal renal function by eliminating damaged mitochondria and minimizing oxidative stress. However, whether astragaloside IV (AS-IV) can play a protective role through the mitophagy pathway is not known.

Methods

We constructed a radiation injury model using hematoxylin and eosin (HE) staining, blood biochemical analysis, immunohistochemistry, TdT-mediated dUTP nick end labeling (TUNEL) staining, ultrastructural observation, and Western blot analysis to elucidate the AS-IV resistance mechanism for IR-induced renal injury.

Results

IR induced mitochondrial damage; the increase of creatinine (SCr), blood urea nitrogen (BUN) and uric acid (UA); and the activation of NOD-like receptor thermal protein domain-associated protein 3 (NLRP3) inflammasome and apoptosis in renal tissue. AS-IV administration attenuated the IR-induced MD and reactive oxygen species (ROS) levels in the kidney; enhanced the levels of mitophagy-associated protein [PTEN-induced putative kinase 1 (PINK1)], parkin proteins, and microtubule-associated protein 1 light 3 (LC3) II/I ratio in renal tissues; diminished NLRP3 inflammasome activation-mediated proteins [cleaved cysteinyl aspartate-specific proteinase-1 (caspase-1), interleukin-1β (IL-1β)] and apoptosis-related proteins [cleaved caspase-9, cleaved caspase-3, BCL2-associated X (Bax)]; reduced SCr, BUN, and UA levels; and attenuated the histopathological alterations in renal tissue. Conversely, mitophagy inhibitor cyclosporin A (CsA) suppressed the AS-IV-mediated protection of renal tissue.

Conclusions

AS-IV can strongly diminish the activation and apoptosis of NLRP3 inflammasome, thus attenuating the renal injury induced by radiation by promoting the PINK1/parkin-mediated mitophagy. These findings suggest that AS-IV is a promising drug for treating IR-induced kidney injury.

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.

Radiation-Induced Bystander Effect and Cytoplasmic Irradiation Studies with Microbeams

Simple Summary

Microbeams are useful tools in studies on non-target effects, such as the radiation-induced bystander effect, and responses related to cytoplasmic irradiation. A micrometer or even sub-micrometer-level beam size enables the precise delivery of radiation energy to a specific target. Here we summarize the observations of the bystander effect and the cytoplasmic irradiation-related effect using different kinds of microbeam irradiators as well as discuss the cellular and molecular mechanisms that are involved in these responses. Non-target effects may increase the detrimental effect caused by radiation, so a more comprehensive knowledge of the process will enable better evaluation of the damage resulting from irradiation.

Abstract

Although direct damage to nuclear DNA is considered as the major contributing event that leads to radiation-induced effects, accumulating evidence in the past two decades has shown that non-target events, in which cells are not directly irradiated but receive signals from the irradiated cells, or cells irradiated at extranuclear targets, may also contribute to the biological consequences of exposure to ionizing radiation. With a beam diameter at the micrometer or sub-micrometer level, microbeams can precisely deliver radiation, without damaging the surrounding area, or deposit the radiation energy at specific sub-cellular locations within a cell. Such unique features cannot be achieved by other kinds of radiation settings, hence making a microbeam irradiator useful in studies of a radiation-induced bystander effect (RIBE) and cytoplasmic irradiation. Here, studies on RIBE and different responses to cytoplasmic irradiation using microbeams are summarized. Possible mechanisms related to the bystander effect, which include gap-junction intercellular communications and soluble signal molecules as well as factors involved in cytoplasmic irradiation-induced events, are also discussed.

Molecular Interactions of Normal and Irradiated Tubulins During Polymerization

Microtubules, one of the cytoskeletons, are highly dynamic structures that play a variety of roles in maintaining cell morphology, cell division and intracellular transport. Microtubules are composed of heterodimers of α- and β-tubulins, which are repeatedly polymerized and depolymerized. To investigate the radiation-induced impacts on the polymerization reaction of tubulins, we evaluated the molecular interactions between normal and irradiated tubulins. First, the polymerization reaction of the tubulins was measured after stepwise irradiation from 0 Gy to 1,000 Gy of X rays. The polymerization was inhibited in a dose-dependent manner. Next, the tubulins' polymerization reaction was then measured after the tubulin that was damaged from the exposure to 1,000 Gy of X rays was mixed with the normal tubulins. Our findings reveal that the radiation dose-dependent change in the degree of overall microtubule polymerization progression depends on the ratio of damaged tubulin. This result is biochemical evidence that non-DNA damage (in this case, cytoskeletal damage) from cytoplasmic radiation exposure may inhibit cell division, suggesting that some cytoskeletal damage may also affect the fate of the entire cell.

Mitochondrial Damage Response and Fate of Normal Cells Exposed to FLASH Irradiation with Protons

Radiation therapy (RT) plays an important role in cancer treatment. The clinical efficacy of radiation therapy is, however, limited by normal tissue toxicity in areas surrounding the irradiated tumor. Compared to conventional radiation therapy (CONV-RT) in which doses are typically delivered at dose rates between 0.03-0.05 Gy/s, there is evidence that radiation delivered at dose rates of orders of magnitude higher (known as FLASH-RT), dramatically reduces the adverse side effects in normal tissues while achieving similar tumor control. The present study focused on normal cell response and tested the hypothesis that proton-FLASH irradiation preserves mitochondria function of normal cells through the induction of phosphorylated Drp1. Normal human lung fibroblasts (IMR90) were irradiated under ambient oxygen concentration (21%) with protons (LET = 10 keV/μm) delivered at dose rates of either 0.33 Gy/s or 100 Gy/s. Mitochondrial dynamics, functions, cell growth and changes in protein expression levels were investigated. Compared to lower dose-rate proton irradiation, FLASH-RT prevented mitochondria damage characterized by morphological changes, functional changes (membrane potential, mtDNA copy number and oxidative enzyme levels) and oxyradical production. After CONV-RT, the phosphorylated form of Dynamin-1-like protein (p-Drp1) underwent dephosphorylation and aggregated into the mitochondria resulting in mitochondria fission and subsequent cell death. In contrast, p-Drp1 protein level did not significantly change after delivery of similar FLASH doses. Compared with CONV irradiation, FLASH irradiation using protons induces minimal mitochondria damage; our results highlight a possible contribution of Drp1-mediated mitochondrial homeostasis in this potential novel cancer treatment modality.

Revisiting the Radiobiology of Targeted Alpha Therapy

Targeted alpha therapy (TAT) using alpha particle-emitting radionuclides is in the spotlight after the approval of ²²³RaCl₂ for patients with metastatic castration-resistant prostate cancer and the development of several alpha emitter-based radiopharmaceuticals. It is acknowledged that alpha particles are highly cytotoxic because they produce complex DNA lesions. Hence, the nucleus is considered their critical target, and many studies did not report any effect in other subcellular compartments. Moreover, their physical features, including their range in tissues (<100 μm) and their linear energy transfer (50–230 keV/μm), are well-characterized. Theoretically, TAT is indicated for very small-volume, disseminated tumors (e.g., micrometastases, circulating tumor cells). Moreover, due to their high cytotoxicity, alpha particles should be preferred to beta particles and X-rays to overcome radiation resistance. However, clinical studies showed that TAT might be efficient also in quite large tumors, and biological effects have been observed also away from irradiated cells. These distant effects are called bystander effects when occurring at short distance (<1 mm), and systemic effects when occurring at much longer distance. Systemic effects implicate the immune system. These findings showed that cells can die without receiving any radiation dose, and that a more complex and integrated view of radiobiology is required. This includes the notion that the direct, bystander and systemic responses cannot be dissociated because DNA damage is intimately linked to bystander effects and immune response. Here, we provide a brief overview of the paradigms that need to be revisited.

Medicinal Thiols: Current Status and New Perspectives

The thiol (-SH) functional group is found in a number of drug compounds and confers a unique combination of useful properties. Thiol-containing drugs can reduce radicals and other toxic electrophiles, restore cellular thiol pools, and form stable complexes with heavy metals such as lead, arsenic, and copper. Thus, thiols can treat a variety of conditions by serving as radical scavengers, GSH prodrugs, or metal chelators. Many of the compounds discussed here have been in use for decades, yet continued exploration of their properties has yielded new understanding in recent years, which can be used to optimize their clinical application and provide insights into the development of new treatments. The purpose of this narrative review is to highlight the biochemistry of currently used thiol drugs within the context of developments reported in the last five years. More specifically, this review focuses on thiol drugs that represent the standard of care for their associated conditions, including N-acetylcysteine, 2,3-meso-dimercaptosuccinic acid, British anti-Lewisite, D-penicillamine, amifostine, and others. Reports of novel dosing regimens, delivery strategies, and clinical applications for these compounds were examined with an eye toward emerging approaches to address a wide range of medical conditions in the future.

DR5 related autophagy can promote apoptosis in gliomas after irradiation

As a cancer treatment strategy, irradiation therapy is widely used that can cause DNA breakage and increase free radicals, which leads to different types of cell death. Among them, apoptosis and autophagy are the most important and the most studied cell death processes. Although the exploration of the relationship between apoptosis and autophagy has been a major area of focus, still the molecular mechanisms of autophagy on apoptosis remain unclear. Here, we have revealed that apoptosis was enhanced by the death receptor 5 (DR5) pathway, and the effect of autophagy on apoptosis was promoted by DR5 interacting with LC3B as well as Caspase8 in gliomas after irradiation. Interestingly, we observed that the addition of four different autophagy inducers, rapamycin (RAP), CCI779, ABT737 and temozolomide (TMZ), induced the differences of DR5 expression and cell apoptosis after irradiation. Unlike RAP and CCI779, ABT737 and TMZ were able to increase DR5 expression and further induce cell death. Therefore, we have concluded that DR5 plays a novel and indispensable role in promoting cell apoptosis under irradiation and suggest a potential therapeutic approach for glioblastoma treatment.

Tetramethylpyrazine Prevents Contrast-Induced Nephropathy via Modulating Tubular Cell Mitophagy and Suppressing Mitochondrial Fragmentation, CCL2/CCR2-Mediated Inflammation, and Intestinal Injury

Contrast-induced nephropathy (CIN) is a leading cause of hospital-acquired acute kidney injury (AKI), but detailed pathogenesis and effectual remedy remain elusive. Here, we tested the hypothesis that contrast media (CM) impaired mitochondrial quality control (MQC) in tubules, including mitochondrial fragmentation and mitophagy, induced systemic inflammation, and intestinal injury. Since we previously demonstrated that the natural antioxidant 2,3,5,6-tetramethylpyrazine (TMP) can be a protectant against CIN, we moreover investigated the involved renoprotective mechanisms of TMP. In a well-established CIN rat model, renal functions, urinary AKI biomarkers, and renal reactive oxygen species (ROS) production were measured. Mitochondrial damage and mitophagy were detected by transmission electron microscopy (TEM) and western blot. The abundance of Drp1 and Mfn2 by western blot and immunohistochemistry (IHC) was used to evaluate mitochondrial fragmentation. TUNEL staining, TEM, and the abundance of cleaved-caspase 3 and procaspase 9 were used to assay apoptosis. We demonstrated that increased mitophagy, mitochondrial fragmentation, ROS generation, autophagy, and apoptosis occurred in renal tubular cells. These phenomena were accompanied by renal dysfunction and an increased excretion of urinary AKI biomarkers. Meanwhile, CM exposure resulted in concurrent small intestinal injury and villous capillary endothelial apoptosis. The abundance of the inflammatory cytokines CCL2 and CCR2 markedly increased in the renal tubules of CIN rats, accompanied by increased concentrations of IL-6 and TNF-α in the kidneys and the serum. Interestingly, TMP efficiently prevented CM-induced kidney injury in vivo by reversing these pathological processes. Mechanistically, TMP inhibited the CM-induced activation of the CCL2/CCR2 pathway, ameliorated renal oxidative stress and aberrant mitochondrial dynamics, and modulated mitophagy in tubular cells. In summary, this study demonstrated novel pathological mechanisms of CIN, that is, impairing MQC, inducing CCL2/CCR2-mediated inflammation and small intestinal injury, and provided novel renoprotective mechanisms of TMP; thus, TMP may be a promising therapeutic agent for CIN.

Biological effects of space environmental factors: A possible interaction between space radiation and microgravity

In the mid-1980s, space experiments began to examine if microgravity could alter the biological effects of space radiation. In the late 1990s, repair of DNA strand breaks was reported to not be influenced by microgravity using the pre-irradiated cells, because the exposure doses of space radiation were few due to the short spaceflight. There were, however, conflicting reports depending on the biological endpoints used in various systems. While almost no attempts were made to assess the possibility that the microgravity effects could be altered by space radiation. This was probably due to the general understanding that microgravity plays a major role in space and works independently from space radiation. Recent ground-based simulation studies focusing on DNA oxidative damage and signal transduction suggested that combined effects of microgravity and space radiation might exist. These studies also implicated the importance of research focusing not only on chromosomal DNA but also on cytoplasm, especially mitochondria. Therefore, we propose a new model which accounts for the combined-effects through the window of cellular responses. In this model, the interactions between microgravity and space radiation might occur during the following cellular-responses; (A) damaging and signaling by ROS, (B) damage responses on DNA (repair, replication, transcription, etc.), and (C) expression of gene and protein (regulation by chromatin, epigenetic control, etc.).

Nuclear factor (erythroid-derived 2)-like 2 antioxidative response mitigates cytoplasmic radiation-induced DNA double-strand breaks

It has been reported that DNA double-strand breaks (DSB) can be induced by cytoplasm irradiation, and that both reactive free radicals and mitochondria are involved in DSB formation. However, the cellular antioxidative responses that are stimulated and the biological consequences of cytoplasmic irradiation remain unknown. Using the Single Particle Irradiation system to Cell (SPICE) proton microbeam facility at the National Institute of Radiological Sciences ([NIRS] Japan), the response of nuclear factor (erythroid-derived 2)-like 2 (NRF2) antioxidative signaling to cytoplasmic irradiation was studied in normal human lung fibroblast WI-38 cells. Cytoplasmic irradiation stimulated the localization of NRF2 to the nucleus and the expression of its target protein, heme oxygenase 1. Activation of NRF2 by tert-butylhydroquinone mitigated the levels of DSB induced by cytoplasmic irradiation. Mitochondrial fragmentation was also promoted by cytoplasmic irradiation, and treatment with mitochondrial division inhibitor 1 (Mdivi-1) suppressed cytoplasmic irradiation-induced NRF2 activation and aggravated DSB formation. Furthermore, p53 contributed to the induction of mitochondrial fragmentation and activation of NRF2, although the expression of p53 was significantly downregulated by cytoplasmic irradiation. Finally, mitochondrial superoxide (MitoSOX) production was enhanced under cytoplasmic irradiation, and use of the MitoSOX scavenger mitoTEMPOL indicated that MitoSOX caused alterations in p53 expression, mitochondrial dynamics, and NRF2 activation. Overall, NRF2 antioxidative response is suggested to play a key role against genomic DNA damage under cytoplasmic irradiation. Additionally, the upstream regulators of NRF2 provide new clues on cytoplasmic irradiation-induced biological processes and prevention of radiation risks.

Helicobacter pylori Infection Modulates Host Cell Metabolism through VacA-Dependent Inhibition of mTORC1

Helicobacter pylori (Hp) vacuolating cytotoxin (VacA) is a bacterial exotoxin that enters host cells and induces mitochondrial dysfunction. However, the extent to which VacA-dependent mitochondrial perturbations affect overall cellular metabolism is poorly understood. We report that VacA perturbations in mitochondria are linked to alterations in cellular amino acid homeostasis, which results in the inhibition of mammalian target of rapamycin complex 1 (mTORC1) and subsequent autophagy. mTORC1, which regulates cellular metabolism during nutrient stress, is inhibited during Hp infection by a VacA-dependent mechanism. This VacA-dependent inhibition of mTORC1 signaling is linked to the dissociation of mTORC1 from the lysosomal surface and results in activation of cellular autophagy through the Unc 51-like kinase 1 (Ulk1) complex. VacA intoxication results in reduced cellular amino acids, and bolstering amino acid pools prevents VacA-mediated mTORC1 inhibition. Overall, these studies support a model that Hp modulate host cell metabolism through the action of VacA at mitochondria.

Geometrical structures for radiation biology research as implemented in the TOPAS-nBio toolkit

Computational simulations, such as Monte Carlo track structure simulations, offer a powerful tool for quantitatively investigating radiation interactions within cells. The modelling of the spatial distribution of energy deposition events as well as diffusion of chemical free radical species, within realistic biological geometries, can help provide a comprehensive understanding of the effects of radiation on cells. Track structure simulations, however, generally require advanced computing skills to implement. The TOPAS-nBio toolkit, an extension to TOPAS (TOol for PArticle Simulation), aims to provide users with a comprehensive framework for radiobiology simulations, without the need for advanced computing skills. This includes providing users with an extensive library of advanced, realistic, biological geometries ranging from the micrometer scale (e.g. cells and organelles) down to the nanometer scale (e.g. DNA molecules and proteins). Here we present the geometries available in TOPAS-nBio.

Metformin prolongs survival rate in mice and causes increased excretion of cell-free DNA in the urine of X-irradiated rats

An antidiabetic drug metformin has anticarcinogenic and geroprotective effects and has been used in combination with radiation cancer therapy. The present work is devoted to the study of the effect of metformin on survival in mice, the frequency of micronuclei in mouse bone marrow cells and excretion of cell-free nuclear and mitochondrial DNA in the urine of X-ray-exposed rats. The survival rate and the frequency of micronuclei in mice and excretion of DNA into rat urine were determined after administration of the drug before and after irradiation of animals. The DNA content was measured by qRT-PCR. Metformin shows a radioprotective effect only when administered to mice after the radiation exposure. On the 11th day after irradiation, we observed 100% mortality in the control group; 78% of mice remained alive if metformin was given. Twenty percent of the mice in this group survived for 30 days after irradiation. Metformin has the same effect on the frequency of micronuclei; its reduction is observed, when the drug is administered to the mice after irradiation. Metformin promotes the excretion of nuclear and mitochondrial DNA with the urine of irradiated rats. The results show that metformin acts as a radiomitigation effector. Metformin promotes the active excretion of DNA of dying cells from the tissues of irradiated animals.

Focus small to find big - the microbeam story

PURPOSE

Even though the first ultraviolet microbeam was described by S. Tschachotin back in 1912, the development of sophisticated micro-irradiation facilities only began to flourish in the late 1980s. In this article, we highlight significant microbeam experiments, describe the latest microbeam irradiator configurations and critical discoveries made by using the microbeam apparatus.

MATERIALS AND METHODS

Modern radiological microbeams facilities are capable of producing a beam size of a few micrometers, or even tens of nanometers in size, and can deposit radiation with high precision within a cellular target. In the past three decades, a variety of microbeams has been developed to deliver a range of radiations including charged particles, X-rays, and electrons. Despite the original intention for their development to measure the effects of a single radiation track, the ability to target radiation with microbeams at sub-cellular targets has been extensively used to investigate radiation-induced biological responses within cells.

RESULTS

Studies conducted using microbeams to target specific cells in a tissue have elucidated bystander responses, and further studies have shown reactive oxygen species (ROS) and reactive nitrogen species (RNS) play critical roles in the process. The radiation-induced abscopal effect, which has a profound impact on cancer radiotherapy, further reaffirmed the importance of bystander effects. Finally, by targeting sub-cellular compartments with a microbeam, we have reported cytoplasmic-specific biological responses. Despite the common dogma that nuclear DNA is the primary target for radiation-induced cell death and carcinogenesis, studies conducted using microbeam suggested that targeted cytoplasmic irradiation induces mitochondrial dysfunction, cellular stress, and genomic instability. A more recent development in microbeam technology includes application of mouse models to visualize in vivo DNA double-strand breaks.

CONCLUSIONS

Microbeams are making important contributions towards our understanding of radiation responses in cells and tissue models.