The Low-dose Ionizing Radiation Stimulates Cell Proliferation via Activation of the MAPK/ERK Pathway in Rat Cultured Mesenchymal Stem Cells

Light assisted modulation of stem cell function and secretome production: a systematic review on current status and new avenues for regenerative medicine

Stem cells (SC) based therapies are proving to be the mainstay of regenerative medicine. Despite the significant potential, direct grafting or implantation of SCs for regenerative therapy encounters various translational roadblocks such as paucity of implantable cells, decreased potency, cell death post-implantation, cell damage caused by the pre-existing inflammation and immune rejection. Hence, an emerging avenue is cell-free approach; use of SC secretome. Although priming approaches based on pharmacological molecules/chemicals, cytokines and growth factors are being explored to elicit enhanced secretome production, the potential concerns include the need for continuous replenishment and potential chemical contamination during secretome isolation. To alleviate these concerns, various non-pharmacological approaches for invigorating SCs are also being investigated and among these, use of photobiomodulation (PBM) has garnered considerable interest. Notwithstanding the positive outcomes, standardized parameters are yet to be established for reproducible results. Moreover, the mechanisms of PBM based SC stimulation and secretome production are poorly elucidated and significant knowledge gaps exist on influence of cell type, culture conditions on PBM. This review aims to provide insight into the current status of this emerging field emphasizing on novel avenues and potential challenges for clinical translation. We also summarize the studies on PBM based proliferation, differentiation and secretome production according to SC cell type and culture conditions. Further, as a fixed PBM based protocol for SC proliferation, differentiation and secretome is lacking, the knowledge on functional targets and pathways in PBM based SC stimulation needs upgradation. Consequently, putative mechanisms for PBM based SC secretome have been proposed.

Differential biological effect of low doses of ionizing radiation depending on the radiosensitivity in a cell line model

PURPOSE

Exposure to low doses (LD) of ionizing radiation (IR), such as the ones employed in computed tomography (CT) examination, can be associated with cancer risk. However, cancer development could depend on individual radiosensitivity. In the present study, we evaluated the differences in the response to a CT-scan radiation dose of 20 mGy in two lymphoblastoid cell lines with different radiosensitivity.

MATERIALS AND METHODS

Several parameters were studied: gene expression, DNA damage, and its repair, as well as cell viability, proliferation, and death. Results were compared with those after a medium dose of 500 mGy.

RESULTS

After 20 mGy of IR, the radiosensitive (RS) cell line showed an increase in DNA damage, and higher cell proliferation and apoptosis, whereas the radioresistant (RR) cell line was insensitive to this LD. Interestingly, the RR cell line showed a higher expression of an antioxidant gene, which could be used by the cells as a protective mechanism. After a dose of 500 mGy, both cell lines were affected by IR but with significant differences. The RS cells presented an increase in DNA damage and apoptosis, but a decrease in cell proliferation and cell viability, as well as less antioxidant response.

CONCLUSIONS

A differential biological effect was observed between two cell lines with different radiosensitivity, and these differences are especially interesting after a CT scan dose. If this is confirmed by further studies, one could think that individuals with radiosensitivity-related genetic variants may be more vulnerable to long-term effects of IR, potentially increasing cancer risk after LD exposure.

Interplay of immune modulation, adaptive response and hormesis: Suggestive of threshold for clinical manifestation of effects of ionizing radiation at low doses?

The health impacts of low-dose ionizing radiation exposures have been a subject of debate over the last three to four decades. While there has been enough evidence of "no adverse observable" health effects at low doses and low dose rates, the hypothesis of "Linear No Threshold" continues to rule and govern the principles of radiation protection and the formulation of regulations and public policies. In adopting this conservative approach, the role of the biological processes underway in the human body is kept at abeyance. This review consolidates the available studies that discuss all related biological pathways and repair mechanisms that inhibit the progression of deleterious effects at low doses and low dose rates of ionizing radiation. It is pertinent that, taking cognizance of these processes, there is a need to have a relook at policies of radiation protection, which as of now are too stringent, leading to undue economic losses and negative public perception about radiation.

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.

Biophysical cues to improve the immunomodulatory capacity of mesenchymal stem cells: The progress and mechanisms

Mesenchymal stem cells (MSCs) can maintain immune homeostasis and many preclinical trials with MSCs have been carried out around the world. In vitro culture of MSCs has been found to result in the decline of immunomodulatory capacity, migration and proliferation. To address these problems, simulating the extracellular environment for preconditioning of MSCs is a promising and inexpensive method. Biophysical cues in the external environment that MSCs are exposed to have been shown to affect MSC migration, residency, differentiation, secretion, etc. We review the main ways in which MSCs exert their immunomodulatory ability, and summarize recent advances in mechanical preconditioning of MSCs to enhance immunomodulatory capacity and related mechanical signal sensing and transduction mechanisms.

Evaluation of Changes in Some Functional Properties of Human Mesenchymal Stromal Cells Induced by Low Doses of Ionizing Radiation

Each person is inevitably exposed to low doses of ionizing radiation (LDIR) throughout their life. The research results of LDIR effects are ambiguous and an accurate assessment of the risks associated with the influence of LDIR is an important task. Mesenchymal stromal cells (MSCs) are the regenerative reserve of an adult organism; because of this, they are a promising model for studying the effects of LDIR. The qualitative and quantitative changes in their characteristics can also be considered promising criteria for assessing the risks of LDIR exposure. The MSCs from human connective gingiva tissue (hG-MSCs) were irradiated at doses of 50, 100, 250, and 1000 mGy by the X-ray unit RUST-M1 (Russia). The cells were cultured continuously for 64 days after irradiation. During the study, we evaluated the secretory profile of hG-MSCs (IL-10, IDO, IL-6, IL-8, VEGF-A) using an ELISA test, the immunophenotype (CD45, CD34, CD90, CD105, CD73, HLA-DR, CD44) using flow cytometry, and the proliferative activity using the xCelligence RTCA cell analyzer at the chosen time points. The results of study have indicated the development of stimulating effects in the early stages of cultivation after irradiation using low doses of X-ray radiation. On the contrary, the effects of the low doses were comparable with the effects of medium doses of X-ray radiation in the long-term periods of cultivation after irradiation and have indicated the inhibition of the functional activity of MSCs.

Neglected immunoregulation: M2 polarization of macrophages triggered by low-dose irradiation plays an important role in bone regeneration

Current studies have found that low-dose irradiation (IR) can promote bone regeneration. However, mechanism studies of IR-triggered bone regeneration mainly focus on the effects of osteoblasts, neglecting the role of the surrounding immune microenvironment. Here in this study, in vitro proliferation experiments showed that low-dose IR ≤2 Gy could promote the proliferation of bone marrow mesenchymal stem cells (BMSCs), and qRT-PCR assay showed that low-dose IR ≤2 Gy could exert the M2 polarization of Raw264.7 cells, while IR >2 Gy inhibited BMSC proliferation and triggered M1 polarization in Raw264.7 cells. The ALP and mineralized nodules staining showed that low-dose IR ≤2 Gy not only promoted osteoblast mineralization through IR-triggered osteoblast proliferation but also through M2 polarization of Raw264.7 cells, while high-dose IR >2 Gy had the opposite effect. The co-incubation of BMSC with low-dose IR irradiated Raw264.7 cell supernatants increased the mRNA expression of BMP-2 and Osx. The rat cranial defects model revealed that low-dose IR ≤2 Gy gradually promoted bone regeneration, while high-dose IR >2 Gy inhibited bone regeneration. Detection of macrophage polarity in peripheral blood samples showed that low-dose IR ≤2 Gy increased the expression of CD206 and CD163, but decreased the expression of CD86 and CD80 in macrophages, which indicated M2 polarization of macrophages in vivo, while high-dose IR had the opposite effect. Our finding innovatively revealed that low-dose IR ≤2 Gy promotes bone regeneration not only by directly promoting the proliferation of osteoblasts but also by triggering M2 polarization of macrophages, which provided a new perspective for immune mechanism study in the treatment of bone defects with low-dose IR.

Exendin-4 regulates the MAPK and WNT signaling pathways to alleviate the osteogenic inhibition of periodontal ligament stem cells in a high glucose environment

Diabetes mellitus (DM) increases the destruction of periodontal tissue and impairs osteogenesis differentiation. Exendin-4 (Ex-4), a glucagon-like peptide-1 (GLP-1) analogue, can be used for treating DM and promotes bone regeneration. The aim of this study was to explore the effect and mechanism of Ex-4 on improving the osteogenesis of periodontal ligament stem cells (PDLSCs) in a high glucose environment. Alkaline phosphatase staining and alizarin red staining were used to detect the osteogenic differentiation of PDLSCs. The results showed that 10 nM Ex-4 could reduce the osteogenesis inhibition of PDLSCs induced by high glucose. RT-PCR and western blot results showed that Ex-4 increased the osteogenesis-related gene expression of ALP, Runx2, and Osx, and upregulated the phosphorylation of P38, JNK, and ERK1/2; the peak effect was observed in the range 0.5-1.0 h. Mitogen-activated protein kinase (MAPK) inhibitors PD98059, SB203580, and SP600125 blocked the effects of Ex-4 on MAPK activation and decreased the expression of ALP, Runx2, and Osx in PDLSCs. Moreover, after Ex-4 treatment, the total β-catenin, p-GSK3β, LEF, and Runx2 protein levels increased under normal or high glucose environments. In conclusion, our results indicated that Ex-4 regulates the MAPK and WNT signaling pathways to alleviate the osteogenic inhibition of PDLSCs in a high glucose environment.

Role of low-dose radiation in senescence and aging: A beneficial perspective

Cellular senescence refers to the permanent arrest of cell cycle caused by intrinsic and/or extrinsic stressors including oncogene activation, irradiation, DNA damage, oxidative stress, and certain cytokines (including senescence associated secretory phenotype). Cellular senescence is an important factor in aging. Accumulation of senescent cells has been implicated in the causation of various age-related organ disorders, tissue dysfunction, and chronic diseases. It is widely accepted that the biological effects triggered by low-dose radiation (LDR) are different from those caused by high-dose radiation. Experimental evidence suggests that LDR may promote growth and development, enhance longevity, induce embryo production, and delay the progression of chronic diseases. The underlying mechanisms of these effects include modulation of immune response, stimulation of hematopoietic system, antioxidative effect, reduced DNA damage and improved ability for DNA damage repair. In this review, we discuss the possible mechanisms by which LDR prevents senescence and aging from the perspectives of inhibiting cellular senescence and promoting the removal of senescent cells. We review a wide broad of evidence about the beneficial impact of LDR in senescence and aging models (including cardiovascular diseases, neurological diseases, arthritis and osteoporosis, chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis) to highlight the potential value of LDR in preventing aging and age-related diseases. However, there is no consensus on the effect of LDR on human health, and several important aspects require further investigation.

A Conserved Mechanism for Hormesis in Molecular Systems

Hormesis refers to dose-response phenomena where low dose treatments elicit a response that is opposite the response observed at higher doses. Hormetic dose-response relationships have been observed throughout all of biology, but the underlying determinants of many reported hormetic dose-responses have not been identified. In this report, we describe a conserved mechanism for hormesis on the molecular level where low dose treatments enhance a response that becomes reduced at higher doses. The hormetic mechanism relies on the ability of protein homo-multimers to simultaneously interact with a substrate and a competitor on different subunits at low doses of competitor. In this case, hormesis can be observed if simultaneous binding of substrate and competitor enhances a response of the homo-multimer. We characterized this mechanism of hormesis in binding experiments that analyzed the interaction of homotrimeric proliferating cell nuclear antigen (PCNA) with uracil DNA glycosylase (UNG2) and a fluorescein-labeled peptide. Additionally, the basic features of this molecular mechanism appear to be conserved with at least two enzymes that are stimulated by low doses of inhibitor: dimeric BRAF and octameric glutamine synthetase 2 (GS2). Identifying such molecular mechanisms of hormesis may help explain specific hormetic responses of cells and organisms treated with exogenous compounds.

Response of murine neural stem/progenitor cells to gamma-neutron radiation

PURPOSE

In recent years, a growing number of studies have focused on the mechanisms of action of densely ionizing radiation. This is associated with the development of radiation therapy of tumors using accelerated ions. The use of densely ionizing radiation appears to be the most promising method, optimal for treating patients with severe radioresistant forms, such as widespread head and neck tumors, recurrent and metastatic tumors, and some forms of brain tumors. The goal of our study was to investigate the effects of gamma-neutron radiation on mouse neural stem/progenitor cells (NSCs/NPCs).

METHODS

NSCs/NPCs were isolated from neonatal mouse brains. Cells were irradiated in a collimated beam of neutrons and gamma rays of the IR-8 nuclear reactor. At 5 and 7 days after irradiation, cells and neurospheres were counted to assess survival. The number of DNA double-strand breaks and their repair efficiency were determined by immunocytochemical γH2AX staining followed by counting the number of γH2AX foci using a fluorescent microscope.

RESULTS

We observed a dose-dependent decrease in the survival of NSCs/NPCs after irradiation at doses above 100 mGy and stimulation of the proliferation of these cells at doses of 25 and 50 mGy. In terms of a decrease in cell survival, the effect of gamma-neutron irradiation significantly exceeded the effect of gamma irradiation: the maximum value of the relative biological efficiency for gamma-neutron irradiation comprised 9.7. Gamma-neutron irradiation led to the formation of double-strand DNA breaks detected by the formation of foci of histone γH2AX in the cell nuclei. The γH2AX foci formed after gamma-neutron irradiation of NSCs/NPCs at doses of 100-500 mGy were characterized by a larger size in comparison with foci induced by gamma irradiation and gamma-neutron irradiation at a dose of 50 mGy. The repair of double-strand DNA breaks induced by γ,n-irradiation was slow; the repair rate depended on the radiation dose.

CONCLUSIONS

The data obtained indicate high sensitivity of proliferating NSCs/NPCs to gamma-neutron radiation. High RBE of gamma-neutron radiation requires special measures to protect the neurogenic regions of the brain when using this type of radiation in radiation therapy.

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.

COVID-19 and low-dose radiation therapy

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative pathogen of the coronavirus disease 2019 (COVID-19), has caused more than 179 million infections and 3.8 million deaths worldwide. Global health authorities working on the COVID-19 outbreak continue to explore methods to reduce the rate of its transmission to healthy individuals. Treatment protocols thus far have focused on social distancing and masking, treatment with antivirals early in infection, and steroids to reduce the inflammatory response. An alternative approach is therapy with low dose radiation (LDR), which has several advantages compared to the current drugs and medicines. To date more than 10 case reports and pilot clinical trial preliminary outcome are available from different countries. These reports cover a wide range of patient conditions and LDR treatment strategies. Although one report showed the failure to observe the improvement of COVID-19 patients after LDR therapy, the majority showed some clinical improvement, and demonstrated the safety of LDR for COVID-19 patients, particularly with 0.5 ​Gy. This review aims to summarize the potential rationales and mechanisms of LDR therapy for COVID-19 patients, and its current clinical status and potential use.

Human exposure to low dose ionizing radiation affects miR-21 and miR-625 expression levels

BACKGROUND

Recently exposure to ionizing radiation driven by artificial radiation sources such as Medical X-rays and Nuclear medicine has increased hastily. Ionizing radiation-induced the DNA damage and activate the DNA damage response signaling pathways. The aim of this study was to evaluate the role of miR-21 and miR-625 in response to low-dose ionizing radiation.

MATERIALS AND METHODS

In this study, the blood sample of 38 volunteer patients who underwent Cardiac scans before and after ^99mTc-MIBI injection were used. The WBC of patients was used for RNA extraction and after cDNA synthesis by the poly-A method the expression level of miR-21 and miR-625 was evaluated by real-time PCR method.

RESULTS

The results of this study indicated that miR-21 and miR- 625 were significantly upregulated under exposure to low-dose ionizing radiation. The expression level of these miRNAs was not significantly correlated with the age and BMI of patients. More ever the bioinformatics analysis indicated that SP1 was a common target of both miRNAs and had the highest degree between hub genes.

CONCLUSION

In summary miR-21 and miR-625 can contribute to the response to acute low dose ionizing radiation by targeting the SP1. However further studies should be carried out on the molecular mechanism of effects of miR-21 and miR-625 in response to low dose ionizing radiation by targeting the SP1.

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.

Hormesis and adult adipose-derived stem cells

This paper provides a detailed assessment of the occurrence of hormetic dose responses in adipose-derived stem cells (ADSCs) of animal models and humans. While a broad range of endpoints has been considered, the predominant research focus in the literature has involved cell proliferation and differentiation. Hormetic dose responses have been commonly reported for ADSCs, encompassing a broad range of chemicals, including pharmaceuticals, dietary supplements and endogenous agents as well as a broad range of physical stressors such as low frequency vibrations, electromagnetic frequency (EMF), heat and sound waves. Numerous agents upregulate key functions such as cell proliferation and differentiation in ADSCs, following the quantitative features of the hormesis dose response model. The paper also assesses the capacity of agents to selectively and dose-dependently activate cell proliferation and/or differentiation, their underlying mechanistic foundations and potential clinical implications. These findings indicate that hormetic dose responses are a prominent feature of ADSC biology and may have a determinant role in their potential clinical applications.

In Vitro Radiosensitivity of Murine Marrow Stromal Cells Varies Across Donor Strains

Mouse models are widely used in the study of musculoskeletal radiobiology both in vivo and in vitro. Two of the most commonly used mouse strains are C57BL/6 and BALB/c. However, little is known about their equivalence in response to ionizing radiation. In this study we compare the responses of marrow stromal cells derived from both of these strains to X rays in vitro at passages 0 and 2. Colony-forming efficiency was significantly higher in BALB/c marrow stromal cells at passage 0. Radiation-induced decreases in colony-forming unit (CFU) formation at passage 0 were comparable across both strains at 0-2 Gy, but BALB/c stromal cells were more radiosensitive than C57BL/6 stromal cells at 3-7 Gy. Osteogenic differentiation at passage 2 was not affected by radiation for either strain. This work demonstrates that commonly used inbred mouse strains differ in their early-passage marrow stromal cell responses to X rays, including self-renewal and differentiation potential. This variability is an important point to consider when selecting an animal model for in vivo or in vitro study.

The Particle Radiobiology of Multipotent Mesenchymal Stromal Cells: A Key to Mitigating Radiation-Induced Tissue Toxicities in Cancer Treatment and Beyond?

Mesenchymal stromal cells (MSCs) comprise a heterogeneous population of multipotent stromal cells that have gained attention for the treatment of irradiation-induced normal tissue toxicities due to their regenerative abilities. As the vast majority of studies focused on the effects of MSCs for photon irradiation-induced toxicities, little is known about the regenerative abilities of MSCs for particle irradiation-induced tissue damage or the effects of particle irradiation on the stem cell characteristics of MSCs themselves. MSC-based therapies may help treat particle irradiation-related tissue lesions in the context of cancer radiotherapy. As the number of clinical proton therapy centers is increasing, there is a need to decidedly investigate MSC-based treatments for particle irradiation-induced sequelae. Furthermore, therapies with MSCs or MSC-derived exosomes may also become a useful tool for manned space exploration or after radiation accidents and nuclear terrorism. However, such treatments require an in-depth knowledge about the effects of particle radiation on MSCs and the effects of MSCs on particle radiation-injured tissues. Here, the existing body of evidence regarding the particle radiobiology of MSCs as well as regarding MSC-based treatments for some typical particle irradiation-induced toxicities is presented and critically discussed.

Mesenchymal Stem Cells Early Response to Low-Dose Ionizing Radiation

Introduction

Mesenchymal stem cells (MSCs) are applied as the therapeutic agents, e.g., in the tumor radiation therapy.

Purpose of the Study

To evaluate the human adipose MSC early response to low-dose ionizing radiation (LDIR).

Materials and Methods

We investigated different LDIR (3, 10, and 50 cGy) effects on reactive oxygen species production, DNA oxidation (marker 8-oxodG), and DNA breaks (marker ɣ H2AX) in the two lines of human adipose MSC. Using reverse transcriptase-polymerase chain reaction, fluorescence-activated cell sorting, and fluorescence microscopy, we determined expression of genes involved in the oxidative stress development (NOX4), antioxidative response (NRF2), antiapoptotic and proapoptotic response (BCL2, BCL2A1, BCL2L1, BIRC2, BIRC3, and BAX1), in the development of the nuclear DNA damage response (DDR) (BRCA1, BRCA2, ATM, and P53). Cell cycle changes were investigated by genes transcription changes (CCND1, CDKN2A, and CDKN1A) and using proliferation markers KI-67 and proliferating cell nuclear antigen (PCNA).

Results

Fifteen to 120 min after exposure to LDIR in MSCs, transient oxidative stress and apoptosis of the most damaged cells against the background of the cell cycle arrest were induced. Simultaneously, DDR and an antiapoptotic response were found in other cells of the population. The 10-cGy dose causes the strongest and fastest DDR following cell nuclei DNA damage. The 3-cGy dose induces a less noticeable and prolonged response. The maximal low range dose, 50 cGy, causes a damaging effect on the MSCs.

Conclusion

Transient oxidative stress and the death of a small fraction of the damaged cells are essential components of the MSC population response to LDIR along with the development of DDR and antiapoptotic response. A scheme describing the early MSC response to LDIR is proposed.

Low-dose ionizing radiation as a hormetin: experimental observations and therapeutic perspective for age-related disorders

Hormesis is any kind of biphasic dose-response when low doses of some agents are beneficial while higher doses are detrimental. Radiation hormesis is the most thoroughly investigated among all hormesis-like phenomena, in particular in biogerontology. In this review, we aimed to summarize research evidence supporting hormesis through exposure to low-dose ionizing radiation (LDIR). Radiation-induced longevity hormesis has been repeatedly reported in invertebrate models such as C. elegans, Drosophila and flour beetles and in vertebrate models including guinea pigs, mice and rabbits. On the contrary, suppressing natural background radiation was repeatedly found to cause detrimental effects in protozoa, bacteria and flies. We also discussed here the possibility of clinical use of LDIR, predominantly for age-related disorders, e.g., Alzheimer's disease, for which no remedies are available. There is accumulating evidence that LDIR, such as those commonly used in X-ray imaging including computer tomography, might act as a hormetin. Of course, caution should be exercised when introducing new medical practices, and LDIR therapy is no exception. However, due to the low average residual life expectancy in old patients, the short-term benefits of such interventions (e.g., potential therapeutic effect against dementia) may outweigh their hypothetical delayed risks (e.g., cancer). We argue here that assessment and clinical trials of LDIR treatments should be given priority bearing in mind the enormous economic, social and ethical implications of potentially-treatable, age-related disorders.

Effects of Low-Dose X-Ray on Cell Growth, Membrane Permeability, DNA Damage and Gene Transfer Efficiency

Background

We aimed to reveal if low dose X-rays would induce harmful or beneficial effect or dual response on biological cells and whether there are conditions the radiation can enhance gene transfer efficiency and promote cell growth but without damage to the cells.

Method

A systematic study was performed on the effects of Kilo-V and Mega-V X-rays on the cell morphology, viability, membrane permeability, DNA damage, and gene transfection of 293 T and CHO cells.

Results

The Kilo-V X-rays of very low doses from 0.01 to 0.04 Gray in principle didn't induce any significant change in cell morphology, growth, membrane permeability, and cause DNA damage. The Mega-V X-ray had a damage threshold between 1.0 and 1.5 Gray. The 0.25 Gray Mega-V-X-ray could promote cell growth and gene transfer, while the 1.5 Gray Mega-V X-ray damaged cells.

Conclusion

The very low dose of KV X-rays is safe to cells, while the effects of Mega-V-X-rays are dose-dependent. Mega-V-X-rays with a dose higher than the damage threshold would be harmful, that between 1.0 -1.5 Gray can evoke dual effects, whereas 0.25 Gray MV X-ray is beneficial for both cell growth and gene transfer, thus would be suitable for radiation-enhanced gene transfection.

The Influence of Radiation on Bone and Bone Cells-Differential Effects on Osteoclasts and Osteoblasts

The bone is a complex organ that is dependent on a tight regulation between bone formation by osteoblasts (OBs) and bone resorption by osteoclasts (OCs). These processes can be influenced by environmental factors such as ionizing radiation (IR). In cancer therapy, IR is applied in high doses, leading to detrimental effects on bone, whereas radiation therapy with low doses of IR is applied for chronic degenerative and inflammatory diseases, with a positive impact especially on bone homeostasis. Moreover, the effects of IR are of particular interest in space travel, as astronauts suffer from bone loss due to space radiation and microgravity. This review summarizes the current state of knowledge on the effects of IR on bone with a special focus on the influence on OCs and OBs, as these cells are essential in bone remodeling. In addition, the influence of IR on the bone microenvironment is discussed. In summary, the effects of IR on bone and bone remodeling cells strongly depend on the applied radiation dose, as differential results are provided from in vivo as well as in vitro studies with varying doses of IR. Furthermore, the isolated effects of IR on a single cell type are difficult to determine, as the bone cells and bone microenvironment are building a tightly regulated network, influencing on one another. Therefore, future research is necessary in order to elucidate the influence of different bone cells on the overall radiation-induced effects on bone.

First Insights into the Effect of Low-Dose X-Ray Irradiation in Adipose-Derived Stem Cells

(1) Background: Emerging interest of physicians to use adipose-derived stem cells (ADSCs) for regenerative therapies and the fact that low-dose irradiation (LD-IR ≤ 0.1 Gy) has been reported to enhance the proliferation of several human normal and bone-marrow stem cells, but not that of tumor cells, lead to the idea of improving stem cell therapies via low-dose radiation. Therefore, the aim of this study was to investigate unwanted side effects, as well as proliferation-stimulating mechanisms of LD-IR on ADSCs. (2) Methods: To avoid donor specific effects, ADSCs isolated from mamma reductions of 10 donors were pooled and used for the radiobiological analysis. The clonogenic survival assay was used to classify the long-term effects of low-dose radiation in ADSCs. Afterwards, cytotoxicity and genotoxicity, as well as the effect of irradiation on proliferation of ADSCs were investigated. (3) Results: LD (≤ 0.1 Gy) of ionizing radiation promoted the proliferation and survival of ADSCs. Within this dose range neither geno- nor cytotoxic effects were detectable. In contrast, greater doses within the dose range of >0.1–2.0 Gy induced residual double-strand breaks and reduced the long-term survival, as well as the proliferation rate of ADSCs. (4) Conclusions: Our data suggest that ADSCs are resistant to LD-IR. Furthermore, LD-IR could be a possible mediator to improve approaches of stem cells in the field of regenerative medicine.

DNA damage response manages cell cycle restriction of senile multipotent mesenchymal stromal cells

Multipotent mesenchymal stromal cells (MMSCs) are promising to treat a variety of traumatic and degenerative diseases. However, in vitro-passage aging induces cell cycle arrest and a series of genetic and biological changes, which greatly limits ex vivo cell number expansion and further clinical application of MMSCs. In most cases, DNA damage and DNA damage response (DDR) act as the main cause and executor of cellular senescence respectively. Mechanistically, DNA damage signals induce cell cycle arrest and DNA damage repair via DDR. If the DNA damage is indelible, MMSCs would entry into a permanent cell cycle arrest. It should be noted that apart from DDR signaling, certain proliferation or metabolism pathways are also occupied in DNA damage related cell cycle arrest. New findings of these aspects will also be summarized in this study. In summary, we aim to provide a comprehensive review of DDR associated cell cycle regulation and other major molecular signaling in the senescence of MMSCs. Above knowledge could contribute to improve the limited capacity of in vitro expansion of MMSCs, and then promote their clinical applications.

Oxidized Cell-Free DNA Is a Factor of Stress Signaling in Radiation-Induced Bystander Effects in Different Types of Human Cells

In pathology or under damaging conditions, the properties of cell-free DNA (cfDNA) change. An example of such change is GC enrichment, which drastically alters the biological properties of cfDNA. GC-rich cfDNA is a factor of stress signaling, whereas genomic cfDNA is biologically inactive. GC-rich cfDNA stimulates TLR9-MyD88-NF-κB signaling cascade, leading to an increase in proinflammatory cytokine levels in the organism. In addition, GC-rich DNA is prone to oxidation and oxidized cfDNA can stimulate secondary oxidative stress. This article is a review of works dedicated to the investigation of a low-dose ionizing radiation effect, a bystander effect, and the role of cfDNA in both of these processes.

The Role of Mesenchymal Stem Cells in Radiation-Induced Lung Fibrosis

Radiation therapy is one of the most important treatment modalities for thoracic tumors. Despite significant advances in radiation techniques, radiation-induced lung injury (RILI) still occurs in up to 30% of patients undergoing thoracic radiotherapy, and therefore remains the main dose-limiting obstacle. RILI is a potentially lethal clinical complication of radiotherapy that has 2 main stages: an acute stage defined as radiation pneumonitis, and a late stage defined as radiation-induced lung fibrosis. Patients who develop lung fibrosis have a reduced quality of life with progressive and irreversible organ malfunction. Currently, the most effective intervention for the treatment of lung fibrosis is lung transplantation, but the lack of available lungs and transplantation-related complications severely limits the success of this procedure. Over the last few decades, advances have been reported in the use of mesenchymal stem cells (MSCs) for lung tissue repair and regeneration. MSCs not only replace damaged lung epithelial cells but also promote tissue repair through the secretion of anti-inflammatory and anti-fibrotic factors. Here, we present an overview of MSC-based therapy for radiation-induced lung fibrosis, focusing in particular on the molecular mechanisms involved and describing the most recent preclinical and clinical studies carried out in the field.

The response of human mesenchymal stem cells to internal exposure to tritium β-rays

There is no doubt that estimating the exposure risk of external and internal low-dose radiation is an imperative issue in radiobiological study. Human mesenchymal stem cells (hMSCs) are multipotent and self-renewing, supporting the regeneration of damaged tissue, including tissue damaged by radiation. However, the responses of hMSCs to internal exposure to radionuclides are still insufficiently understood. In order to evaluate the adverse effects produced by internal exposure to tritiated water (HTO) at a low dose, hMSCs were exposed to 2 × 107 Bq/ml HTO, and the biological effects after the exposure were examined. Apoptosis and DNA double-strand breaks (DSBs) were assayed to analyze the cellular response to the damage induced by HTO. Slight enhancement of apoptosis was found after treatment, except at the dose of 9 mGy. The number of DSBs at 24 h post-irradiation showed that the DNA damage was able to be efficiently repaired by the hMSCs. Moreover, the increasing proportion of the cell population in S phase proved that the persistence of residual γH2AX foci at lower concentrations of HTO was attributable to the secondary production of DSBs in DNA replication. Our work adds to the available data, helping us understand the risk of stem cell transformation due to internal exposure and its correlation with low-dose radiation-induced carcinogenesis.

Research Progress on the Biological Effects of Low-Dose Radiation in China

Human are exposed to ionizing radiation from natural and artificial sources, which consequently poses a possible risk to human health. However, accumulating evidence indicates that the biological effects of low-dose radiation (LDR) are different from those of high-dose radiation (HDR). Low-dose radiation–induced hormesis has been extensively observed in different biological systems, including immunological and hematopoietic systems. Adaptive responses in response to LDR that can induce cellular resistance to genotoxic effects from subsequent exposure to HDR have also been described and researched. Bystander effects, another type of biological effect induced by LDR, have been shown to widely occur in many cell types. Furthermore, the influence of LDR-induced biological effects on certain diseases, such as cancer and diabetes, has also attracted the interest of researchers. Many studies have suggested that LDR has the potential antitumor and antidiabetic complications effects. In addition, the researches on whether LDR could induce stochastic effects were also debated. Studies on the biological effects of LDR in China started in 1970s and considerable progress has been made since. In the present article, we provide an overview of the research progress on the biological effects of LDR in China.

Differential radio-adaptive responses in BALB/c and C57BL/6 mice: pivotal role of calcium and nitric oxide signalling

Purpose: Our earlier studies demonstrated that transient radio-adaptive responses (RAR) in BALB/c mice were due to MAPK hyperactivation. The objective of this study was to determine the time duration of this low dose induced MAPK activation in BALB/c mice and to find out if similar adaptive responses are observed in C57BL/6 mice. Materials and methods: Mice were irradiated with 0.1 Gy priming dose (PD), 2 Gy challenge dose (CD) with an interval of 4 h (P + CD) and radiation induced immunosuppression in splenic lymphocytes was monitored as the endpoint for RAR. Results: Time kinetics following 0.1 Gy demonstrated persistence of MAPK hyperactivation till 48 h. Similar experiments in C57BL/6 mice indicated absence of RAR at 24 h following CD, in spite of MAPK activation which was also confirmed by time kinetics. Therefore, upstream activators of MAPK, viz., reactive oxygen and nitrogen species (ROS, RNS) and calcium levels were estimated. There was increased intracellular calcium (Ca²⁺) and nitric oxide (NO) in BALB/c and an increase in intracellular ROS in C57BL/6 mice 24 h after PD. Inhibition of NO and calcium chelation abrogated RAR in BALB/c mice. In vitro treatment of spleen cells with combination of NO donor and Ca²⁺ ionophore mimicked the effect of PD and induced adaptive response after 2 Gy not only in BALB/c but also in C57BL/6 mice confirming their crucial role in RAR. Conclusions: These results suggest that low dose induced differential induction of Ca²⁺ and NO signaling along with MAPK was responsible for contrasting RAR with respect to immune system of BALB/c and C57BL/6 mice. Abbreviations [³H]-TdR: ³H-methyl-thymidine; BAPTA: 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; CD: Challenge Dose; CFSE: Carboxy Fluorescein Succinamidyl Ester; on A: Concanavalin A; DAF-FM: 4-amino-5-methylamino-2',7'-difluorescein; DCF-DA: 2',7'-dichlorofluorescein diacetate; DSB: Double Strand Break; ELISA: Enzyme Linked ImmunoSorbent Assay; ERK: Extracellular signal-Regulated protein Kinase; FBS: Fetal Bovine Serum; HIF-1A: Hypoxia-Inducible Factor 1-alpha; LDR: Low Dose Radiation; MAPK: Mitogen Activated Protein Kinase; MAPKK/MKK: MAPK Kinase; MAPKKK: MAPK Kinase Kinase; NO: Nitric Oxide; NOS: Nitric Oxide Synthase; P + CD: Priming + Challenge dose; PBS: Phosphate Buffered Saline; PBST: Phosphate Buffered Saline-Tween 20; PD: Priming Dose; PI3K: Phosphatidyl Inositol 3-Kinase; PKC: Protein Kinase C; RAR: Radio Adaptive Response; RNS: Reactive Nitrogen Species; ROS: Reactive Oxygen Species; RPMI-1640: Roswell Park Memorial Institute-1640 medium; SAPK/JNK: Stress-Activated Protein Kinase/ c-Jun NH2-terminal Kinase; SEM: Standard Error of Mean; SNAP: S-nitro amino penicillamine; TP53: Tumor Protein 53; γ-H2AX: Gamma- H2A histone family member X; Th1: Type 1 helper T cell responses; Th2: Type 2 helper T cell responses.

Low dose ionizing irradiation suppresses cellular senescence in normal human fibroblasts

PURPOSE

Exposure to high dose ionizing radiation leads to premature cell senescence and suppression of cell proliferation. In contrast, low dose and low dose-rate gamma-irradiation can lead to stimulation of cell proliferation. We aimed to examine whether the low dose radiation-induced proliferation of normal human fibroblasts can lead to a progressive depletion of proliferation potential and to an early onset of senescence.

MATERIALS AND METHODS

Normal human embryonic lung fibroblasts (HELF-104) at passage 22-24 were gamma-irradiated with doses of 0 (sham-irradiation), 10, 30, 50, 90, 120, 150, 200, and 500 mGy as well as 1 and 2 Gy. After irradiation, the fraction of cells positively stained for senescence-associated β-galactosidase activity was measured weekly until the cell culture completely ceased to proliferate.

RESULTS

We show that single irradiation of HELF-104 cells with 30 and 50 mGy resulted in deceleration of senescence. The suppression of senescence was observed during almost the entire length of the study up to a complete arrest of cell growth.

CONCLUSIONS

Our data, together with the previously published observation of delayed stimulation of proliferation in HELF-104 cells exposed to 30 mGy, suggest that low dose gamma-irradiation can increase the overall proliferative potential of normal human fibroblasts.

Radiation Induces Apoptosis and Osteogenic Impairment through miR-22-Mediated Intracellular Oxidative Stress in Bone Marrow Mesenchymal Stem Cells

Bone marrow mesenchymal stem cells (BMSCs) were characterized by their multilineage potential and were involved in both bony and soft tissue repair. Exposure of cells to ionizing radiation (IR) triggers numerous biological reactions, including reactive oxygen species (ROS), cellular apoptosis, and impaired differentiation capacity, while the mechanisms of IR-induced BMSC apoptosis and osteogenic impairment are still unclear. Through a recent study, we found that 6 Gy IR significantly increased the apoptotic ratio and ROS generation, characterized by ROS staining and mean fluorescent intensity. Intervention with antioxidant (NAC) indicated that IR-induced cellular apoptosis was partly due to the accumulation of intracellular ROS. Furthermore, we found that the upregulation of miR-22 in rBMSCs following 6 Gy IR played an important role on the ROS generation and subsequent apoptosis. In addition, we firstly demonstrated that miR-22-mediated ROS accumulation and cell injury had an important regulated role on the osteogenic capacity of BMSCs both in vitro and in vivo. In conclusion, IR-induced overexpression of miR-22 regulated the cell viability and differentiation potential through targeting the intracellular ROS.

Repeated Whole-Body Exposure to Low-Dose Radiation Combined With Topical Application of Basic Fibroblast Growth Factor and Zinc Accelerates Wound Healing in Diabetic Rats

We reported the acceleration of skin wound healing in diabetic rats by repeated exposure to low-dose radiation (LDR). Here, we explored whether the wound healing could be further improved when LDR was combined with a topical application of basic fibroblast growth factor (bFGF) or zinc. Wounds were established on the backs of type 1 diabetic rats induced by a single injection of streptozotocin. Rats were treated daily with normal saline (Diabetes), LDR, bFGF, zinc, or combined 3 treatments for 5 consecutive days with a 2-day break between each consecutive 5-day treatment. Changes in wound size, histopathology, and microvessel density were assessed on days 5, 10, and 15, respectively, once treatment is started. All treatment regimens significantly accelerated skin wound healing, tissue remodeling, and new vessel formation compared to diabetes group. However, the combined LDR plus bFGF and zinc provided a better beneficial effect on wound healing than either one of these treatments alone. Further, we found that the effects of LDR and bFGF were similar, whereas zinc alone induced a weaker response. Our results suggest that whole-body LDR plus the topical application of bFGF and zinc can further accelerate wound healing in diabetic rats.

Validating the pivotal role of the immune system in low-dose radiation-induced tumor inhibition in Lewis lung cancer-bearing mice

Although low-dose radiation (LDR) possesses the two distinct functions of inducing hormesis and adaptive responses, which result in immune enhancement and tumor inhibition, its clinical applications have not yet been elucidated. The major obstacle that hinders the application of LDR in the clinical setting is that the mechanisms underlying induction of tumor inhibition are unclear, and the risks associated with LDR are still unknown. Thus, to overcome this obstacle and elucidate the mechanisms mediating the antitumor effects of LDR, in this study, we established an in vivo lung cancer model to investigate the participation of the immune system in LDR-induced tumor inhibition and validated the pivotal role of the immune system by impairing immunity with high-dose radiation (HDR) of 1 Gy. Additionally, the LDR-induced adaptive response of the immune system was also observed by sequential HDR treatment in this mouse model. We found that LDR-activated T cells and natural killer cells and increased the cytotoxicity of splenocytes and the infiltration of T cells in the tumor tissues. In contrast, when immune function was impaired by HDR pretreatment, LDR could not induce tumor inhibition. However, when LDR was administered before HDR, the immunity could be protected from impairment, and tumor growth could be inhibited to some extent, indicating the induction of the immune adaptive response by LDR. Therefore, we demonstrated that immune enhancement played a key role in LDR-induced tumor inhibition. These findings emphasized the importance of the immune response in tumor radiotherapy and may help promote the application of LDR as a novel approach in clinical practice.

Bone marrow-derived mesenchymal stromal cells promote colorectal cancer cell death under low-dose irradiation

BACKGROUND

Radiotherapy remains one of the cornerstones to improve the outcome of colorectal cancer (CRC) patients. Radiotherapy of the CRC not only help to destroy cancer cells but also remodel the tumour microenvironment by enhancing tumour-specific tropism of bone marrow-derived mesenchymal stromal cell (BM-MSC) from the peripheral circulation. However, the role of local MSCs and recruited BM-MSC under radiation were not well defined. Indeed, the functions of BM-MSC without irradiation intervention remained controversial in tumour progression: BM-MSC was previously shown to modulate the immune function of major immune cells, resulting in an impaired immunological sensitivity and to induce an increased risk of tumour recurrence. In contrast, it could also secrete various cytokines and possess anticancer effect.

METHODS

Three co-cultivation modules, 3D culture modules, and cancer organoids were established. The induction of cytokines secretion in hBM-MSCs after irradiation was analysed by ELISA array and flow cytometry. AutoMac separator was used to separate hBM-MSC and CRC automatically. Cells from the co-cultured group and the control group were then irradiated by UV-C lamp and X-ray. Proliferation assay and viability assay were performed.

RESULTS

In this study, we show that BM-MSCs can induce the EMT progression of CRC cells in vitro. When irradiated with low doses of ultraviolet radiation and X-rays, BM-MSCs show an anti-tumour effect by secreting certain cytokine (TNF-α, IFN-γ) that lead to the inhibition of proliferation and induction of apoptosis of CRC cells. This was further verified in a 3D culture model of a CRC cell in vitro. Furthermore, irradiation on the co-culture system induced the cleavage of caspase3, and attenuated the phosphorylation of phosphatidylinositol 3-kinase (PI3K)/AKT and extracellular signal-regulated kinase in cancer cells. The signal pathways above might contribute to the cancer cell death.

CONCLUSIONS

Taken together, we show that BM-MSC can potentially promote the effect of radiotherapy in CRC.

Residual γH2AX foci induced by low dose x-ray radiation in bone marrow mesenchymal stem cells do not cause accelerated senescence in the progeny of irradiated cells

Mechanisms underlying the effects of low-dose ionizing radiation (IR) exposure (10-100 mGy) remain unknown. Here we present a comparative study of early (less than 24h) and delayed (up to 11 post-irradiation passages) radiation effects caused by low (80 mGy) vs intermediate (1000 mGy) dose X-ray exposure in cultured human bone marrow mesenchymal stem cells (MSCs). We show that γН2АХ foci induced by an intermediate dose returned back to the control value by 24 h post-irradiation. In contrast, low-dose irradiation resulted in residual γН2АХ foci still present at 24 h. Notably, these low dose induced residual γН2АХ foci were not co-localized with рАТМ foci and were observed predominantly in the proliferating Кi67 positive (Кi67+) cells. The number of γН2АХ foci and the fraction of nonproliferating (Кi67-) and senescent (SA-β-gal+) cells measured at passage 11 were increased in cultures exposed to an intermediate dose compared to unirradiated controls. These delayed effects were not seen in the progeny of cells that were irradiated with low-dose X-rays, although such exposure resulted in residual γН2АХ foci in directly irradiated cells. Taken together, our results support the hypothesis that the low-dose IR induced residual γH2AХ foci do not play a role in delayed irradiation consequences, associated with cellular senescence in cultured MSCs.

Low-Intensity Ultrasound Upregulates the Expression of Cyclin-D1 and Promotes Cellular Proliferation in Human Mesenchymal Stem Cells

Human mesenchymal stem cells (hMSCs) hold great potential for cellular based therapeutics and tissue engineering applications and their expansion is an interesting prospect due to their low availability from in vivo sources. Therefore, this study investigated the effect of continuous-wave low-intensity ultrasound (LIUS) at 5.0-MHz and 14.0-kPa (<20 mW cm^-2 ) on the proliferative capacity, colony-formation efficiency, genetic stability, and differentiation potential of hMSCs. Additionally, potential signaling pathways involved in LIUS-mediated proliferation of hMSCs are studied. Compared to non-stimulated controls, LIUS-treated hMSCs shows a 1.9-fold greater colony-forming efficiency and 2.5-fold higher rate of cell proliferation, respectively. Differential staining and qRT-PCR analysis for selective chondrogenic, osteogenic, and adipogenic markers further confirmed that the LIUS treatment did not impact the multipotency of hMSCs. LIUS-treated hMSCs expressed normal male karyotype. The synthesis of cyclin-D1, a master regulator of cellular proliferation, is upregulated under LIUS and its enhanced mRNA expression under LIUS is noted to be mediated by the activation of both MAPK/ERK and PI3K/AKT pathways. In conclusion, LIUS promotes proliferation and self-renewal capacity of hMSCs.

Low-dose irradiation inhibits proliferation of the p53null type human prostate cancer cells through the ATM/p21 pathway

Low-dose ionizing radiation (LDIR) induces hormesis, exerts an adoptive effect on normal mammalian cells and stimulates cell proliferation; however, this effect is absent in cancer cells. Little is known on the molecular mechanisms underlying this differential response between normal and cancer cells. In the present study, it was demonstrated that the human prostate cancer cell line PC-3 and the normal prostate cell line RWPE-1 exhibited differential biological responses to LDIR. Through cell cycle analyses, it was demonstrated that LDIR inhibited cell growth and arrested the cell cycle at the S and G2/M phases in PC-3 cells, but not in RWPE-1 cells. Using western blotting, it was demonstrated that LDIR at 75 mGy induced the expression of ataxia-telangiectasia mutated (ATM) protein in PC-3 as well as RWPE-1 cells. However, the ATM̸p21 pathway was activated in PC-3, but not in RWPE-1 cells. Although the expression of p53 was not affected by 75 mGy LDIR in RWPE-1 cells, the ATM̸p21 pathway was activated when RWPE-1 cells lost p53 function. In addition, when using ATM inhibitors, the ATM̸p21 pathway was inactivated in both cell lines, and the LDIR-induced cell proliferation inhibition was also abolished. These findings suggested that the ATM/p21 pathway directly participated in the LDIR-induced cell proliferation inhibition in p53null type prostate tumor cells, whereas this mechanism was absent in normal prostate cells. Thus, p53 may affect cell stability following LDIR, and plays a crucial role in regulating the ATM/p21 pathway activated by LDIR.

Carcinogenesis Induced by Low-dose Radiation

Background

Although the effects of high dose radiation on human cells and tissues are relatively well defined, there is no consensus regarding the effects of low and very low radiation doses on the organism. Ionizing radiation has been shown to induce gene mutations and chromosome aberrations which are known to be involved in the process of carcinogenesis. The induction of secondary cancers is a challenging long-term side effect in oncologic patients treated with radiation. Medical sources of radiation like intensity modulated radiotherapy used in cancer treatment and computed tomography used in diagnostics, deliver very low doses of radiation to large volumes of healthy tissue, which might contribute to increased cancer rates in long surviving patients and in the general population. Research shows that because of the phenomena characteristic for low dose radiation the risk of cancer induction from exposure of healthy tissues to low dose radiation can be greater than the risk calculated from linear no-threshold model. Epidemiological data collected from radiation workers and atomic bomb survivors confirms that exposure to low dose radiation can contribute to increased cancer risk and also that the risk might correlate with the age at exposure.

Conclusions

Understanding the molecular mechanisms of response to low dose radiation is crucial for the proper evaluation of risks and benefits that stem from these exposures and should be considered in the radiotherapy treatment planning and in determining the allowed occupational exposures.

Effects of Localized X-Ray Irradiation on Peripheral Nerve Regeneration in Transected Sciatic Nerve in Rats

Low-dose radiation has been used in clinical and experimental models for the prevention of scarring and for fracture healing. There is evidence that low-dose radiation improves the hormesis of various cell types but little is known about its effects on peripheral nerve tissue. In this study, we investigated the beneficial effects of low-dose radiation on the regeneration of transectional peripheral nerve injury in an experimental rat model. Seventy-two male Sprague-Dawley rats received transection injury to the left sciatic nerves, and the nerves were subsequently sutured by epineurium end-to-end anastomosis to restore continuity. Animals were randomly assigned to one of two treatment groups (n = 36/group): 1 Gy X-ray irradiation or control (sham irradiation). Gait analysis, electrophysiological examination and morphological investigations were performed. In addition, Western blot and qRT-PCR were performed to determine the level of vascular endothelial growth factor (VEGF) and growth-associated protein-43 (GAP-43). Content of VEGF and GAP-43 in the regenerated sciatic nerve of the irradiated group was higher than the control group. At 4 to 12 weeks after surgery, the irradiated animals exhibited a significantly improved functional recovery relative to controls. At 12 weeks after surgery, amplitude and conduction velocity of the irradiated group were higher than the control group (P < 0.05). The number of nerve fibers, diameter of axons and morphological structure of the myelin sheath in the irradiated group were superior to those of the control group. These results suggest that low-dose radiation contributed to regeneration and functional recovery after transverse peripheral nerve injury by inducing increased production of VEGF and GAP-43, which promote the axonal regeneration and myelination.

Low-Dose Ionizing γ-Radiation Promotes Proliferation of Human Mesenchymal Stem Cells and Maintains Their Stem Cell Characteristics

Mesenchymal stem cells (MSCs), which are multipotent and have self-renewal ability, support the regeneration of damaged normal tissue. A number of external stimuli promote migration of MSCs into peripheral blood and support their participation in wound healing. In an attempt to harness the potential beneficial effects of such external stimuli, we exposed human MSCs (hMSCs) to one such stimulus-low-dose ionizing radiation (LDIR)-and examined their biological properties. To this end, we evaluated differences in proliferation, cell cycle, DNA damage, expression of surface markers (CD29, CD34, CD90, and CD105), and differentiation potential of hMSCs before and after irradiation with γ-rays generated using a 137CS irradiator. At doses less than 50 mGy, LDIR had no significant effect on the viability or apoptosis of hMSCs. Interestingly, 10 mGy of LDIR increased hMSC viability by 8% (p < 0.001) compared with non-irradiated hMSCs. At doses less than 50 mGy, LDIR did not induce DNA damage, including DNA strand breaks, or cause cellular senescence or cell-cycle arrest. Surface marker expression and in vitro differentiation potential of hMSCs were maintained after two exposures to LDIR at 10 mGy per dose. In conclusion, a two-dose exposure to LDIR at 10 mGy per dose not only facilitates proliferation of hMSCs, it also maintains the stem cell characteristics of hMSCs without affecting their viability. These results provide evidence for the potential of LDIR as an external stimulus for in vitro expansion of hMSCs and application in tissue engineering and regenerative medicine.

γH2AX, 53BP1 and Rad51 protein foci changes in mesenchymal stem cells during prolonged X-ray irradiation

At high exposure levels ionizing radiation is a carcinogen. Little is known about how human stem cells, which are known to contribute to tumorigenesis, respond to prolonged radiation exposures. We studied formation of DNA double strand breaks, accessed as γH2AX and 53BP1 foci, in human mesenchymal stem cells (MSCs) exposed to either acute (5400 mGy/h) or prolonged (270 mGy/h) X-irradiation. We show a linear γH2AX and 53BP1 dose response for acute exposures. In contrast, prolonged exposure resulted in a dose-response curve that had an initial linear portion followed by a plateau. Analysis of Rad51 foci, as a marker of homologous recombination, in cells exposed to prolonged irradiation revealed a threshold in a dose response. Using Ki67 as a marker of proliferating cells, we show no difference in the γH2AX distribution in proliferating vs. quiescent cells. However, Rad51 foci were found almost exclusively in proliferating cells. Concurrent increases in the fraction of S/G2 cells were detected in cells exposed to prolonged irradiation by scoring CENPF-positive cells. Our data suggest that prolonged exposure of MSCs to ionizing radiation leads to cell cycle redistribution and associated activation of homologous recombination. Also, proliferation status may significantly affect the biological outcome, since homologous repair is not activated in resting MSCs.

Injectable Fullerenol/Alginate Hydrogel for Suppression of Oxidative Stress Damage in Brown Adipose-Derived Stem Cells and Cardiac Repair

Stem cell implantation strategy has exhibited potential to treat the myocardial infarction (MI), however, the low retention and survival limit their applications due to the reactive oxygen species (ROS) microenvironment after MI. In this study, the fullerenol nanoparticles are introduced into alginate hydrogel to create an injectable cell delivery vehicle with antioxidant activity. Results suggest that the prepared hydrogels exhibit excellent injectable and mechanical strength. In addition, the fullerenol/alginate hydrogel can effectively scavenge the superoxide anion and hydroxyl radicals. Based on these results, the biological behaviors of brown adipose-derived stem cells (BADSCs) seeded in fullerenol/alginate hydrogel were investigated in the presence of H₂O₂. Results suggest that the fullerenol/alginate hydrogels have no cytotoxicity effects on BADSCs. Moreover, they can suppress the oxidative stress damage of BADSCs and improve their survival capacity under ROS microenvironment via activating the ERK and p38 pathways while inhibiting JNK pathway. Further, the addition of fullerenol can improve the cardiomyogenic differentiation of BADSCs even under ROS microenvironment. To assess its therapeutic effects in vivo, the fullerenol/alginate hydrogel loaded with BADSCs were implanted in the MI area in rats. Results suggest that the fullerenol/alginate hydrogel can effectively decrease ROS level in MI zone, improve the retention and survival of implanted BADSCs, and induce angiogenesis, which in turn promote cardiac functional recovery. Therefore, the fullerenol/alginate hydrogel can act as injectable cell delivery vehicles for cardiac repair.

Effects of Medical Diagnostic Low-dose X Rays on Human Lymphocytes: Mitochondrial Membrane Potential, Apoptosis and Cell Cycle

Low-dose radiation is widely used across the world for the diagnosis of many diseases by means of a variety of imaging technologies. However, the harmful effects of exposure to low-dose radiation during medical examination remain controversial. The authors studied the effects of medical diagnostic low-dose x rays (i.e., 0.03, 0.05, or 0.1 mGy) after an in vitro exposure of human lymphocytes. Cells with no irradiation served as the non-irradiated control group. Three biological indicators were used to determine the effects of medical diagnostic low-dose x rays at 4, 8, 24, 48, and 72 h post-irradiation. These biological endpoints were mitochondrial membrane potential (ΔΨm), cell cycle, and apoptosis. Results indicated no changes in the ΔΨm, number of apoptotic cells, and cell cycle in lymphocytes exposed to these low doses of radiation, as compared to the corresponding non-irradiated lymphocytes at all harvest time-points. These results suggested that there were no harmful effects of the diagnostic low-dose x rays when human lymphocytes were exposed in an in vitro condition.