Involvement of purinergic signaling in cellular response to gamma 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.

T Cells Contribute to Pathological Responses in the Non-Targeted Rat Heart following Irradiation of the Kidneys

Heart disease is a significant adverse event caused by radiotherapy for some cancers. Identifying the origins of radiogenic heart disease will allow therapies to be developed. Previous studies showed non-targeted effects manifest as fibrosis in the non-irradiated heart after 120 days following targeted X-irradiation of the kidneys with 10 Gy in WAG/RijCmcr rats. To demonstrate the involvement of T cells in driving pathophysiological responses in the out-of-field heart, and to characterize the timing of immune cell engagement, we created and validated a T cell knock downrat on the WAG genetic backgrou nd. Irradiation of the kidneys with 10 Gy of X-rays in wild-type rats resulted in infiltration of T cells, natural killer cells, and macrophages after 120 days, and none of these after 40 days, suggesting immune cell engagement is a late response. The radiation nephropathy and cardiac fibrosis that resulted in these animals after 120 days was significantly decreased in irradiated T cell depleted rats. We conclude that T cells function as an effector cell in communicating signals from the irradiated kidneys which cause pathologic remodeling of non-targeted heart.

Out-of-field effects: lessons learned from partial body exposure

Partial body exposure and inhomogeneous dose delivery are features of the majority of medical and occupational exposure situations. However, mounting evidence indicates that the effects of partial body exposure are not limited to the irradiated area but also have systemic effects that are propagated outside the irradiated field. It was the aim of the "Partial body exposure" session within the MELODI workshop 2020 to discuss recent developments and insights into this field by covering clinical, epidemiological, dosimetric as well as mechanistic aspects. Especially the impact of out-of-field effects on dysfunctions of immune cells, cardiovascular diseases and effects on the brain were debated. The presentations at the workshop acknowledged the relevance of out-of-field effects as components of the cellular and organismal radiation response. Furthermore, their importance for the understanding of radiation-induced pathologies, for the discovery of early disease biomarkers and for the identification of high-risk organs after inhomogeneous exposure was emphasized. With the rapid advancement of clinical treatment modalities, including new dose rates and distributions a better understanding of individual health risk is urgently needed. To achieve this, a deeper mechanistic understanding of out-of-field effects in close connection to improved modelling was suggested as priorities for future research. This will support the amelioration of risk models and the personalization of risk assessments for cancer and non-cancer effects after partial body irradiation.

Interferon-α exerts proinflammatory properties in experimental radiation-induced esophagitis: Possible involvement of plasmacytoid dendritic cells

AIMS

Radiation-induced esophagitis, experienced during radiation therapy for lung cancer and head and neck cancer, is a major dose-limiting side effect of the treatment. This study aimed to elucidate the role of interferon-α (IFN-α) in radiation-induced esophagitis.

MAIN METHODS

C57BL/6 mice were exposed to 10 and 25Gy of single thoracic irradiation. Esophageal mucosal damage and inflammatory reactions were assessed for 5 days after irradiation.

KEY FINDINGS

Irradiation induced esophagitis, characterized by reduction in the thickness of epithelial layer, upregulation of proinflammatory cytokines and chemokines, infiltration of inflammatory cells into the esophageal mucosa, and apoptosis of epithelial cells. Irradiation upregulated the level of gene expression for IFN-α in the esophageal tissue, and the neutralizing antibody against IFN-α ameliorated radiation-induced esophageal mucosal damage, while administration of IFN-α receptor agonist (RO8191) had an inverse effect. Depletion of plasmacytoid dendritic cells (pDCs) by anti-CD317 antibody or pharmacological inactivation with bortezomib suppressed radiation-induced mucosal inflammation and damage, accompanied by decrease in IFN-α expression level.

SIGNIFICANCE

These findings suggest that IFN-α and pDCs exert proinflammatory properties in the pathophysiology of radiation-induced esophagitis.

Mass Treatment of Flooded Archival Materials by Gamma Radiation

This work describes mass recovery processes of flooded archival materials at industrial scale. The presence of fungi on paper represents a threat to the integrity of the document because they degrade cellulose, one of the main components of paper. Gamma radiation treatments are investigated as mass disinfection agents for their high penetrating power, speed of treatment, and absence of risk due to chemical residuals. We compared two different recovery processes: thermal drying followed by gamma irradiation and gamma irradiation followed by thermal drying. Both these processes were conducted simultaneously on naturally contaminated archival items and on paper specimens artificially contaminated with test species. Efficacy was assessed by culture method and ATP assay, right after the treatments and after four years of storage at room temperature. Coupling gamma irradiation with a drying step with dry heat at 55–60 °C reduces the fungal loads on natural items up to levels close to the detection limits, and the reduction is maintained after four years. On artificial specimens, spore germination is completely inhibited, mycelia growth is also highly affected, but the melanised test species appear to be more resistant. A synergistic effect between gamma irradiation, water content, and thermal drying is highlighted in this paper.

Purinergic Signaling Within the Tumor Microenvironment

Accumulating studies have clearly demonstrated high concentrations of extracellular ATP (eATP) within the tumor microenvironment (TME). Implications of these findings are multifold as ATP-mediated purinergic signaling has been shown to mediate a variety of cancer-related processes, including cell migration, resistance to cytotoxic therapy, and immune regulation. Broad roles of ATP within the tumor microenvironment are linked to the abundance of ATP-regulated purinergic receptors on cancer and stromal and various immune cell types, as well as on the importance of ATP release and signaling in the regulation of multiple cellular processes. ATP release and downstream purinergic signaling are emerging as a central regulator of tumor growth and an important target for therapeutic intervention. In this chapter, we summarize the major roles of purinergic signaling in the tumor microenvironment with a specific focus on its critical roles in the induction of immunogenic cancer cell death and immune modulation.

The role of connexin proteins and their channels in radiation-induced atherosclerosis

Radiotherapy is an effective treatment for breast cancer and other thoracic tumors. However, while high-energy radiotherapy treatment successfully kills cancer cells, radiation exposure of the heart and large arteries cannot always be avoided, resulting in secondary cardiovascular disease in cancer survivors. Radiation-induced changes in the cardiac vasculature may thereby lead to coronary artery atherosclerosis, which is a major cardiovascular complication nowadays in thoracic radiotherapy-treated patients. The underlying biological and molecular mechanisms of radiation-induced atherosclerosis are complex and still not fully understood, resulting in potentially improper radiation protection. Ionizing radiation (IR) exposure may damage the vascular endothelium by inducing DNA damage, oxidative stress, premature cellular senescence, cell death and inflammation, which act to promote the atherosclerotic process. Intercellular communication mediated by connexin (Cx)-based gap junctions and hemichannels may modulate IR-induced responses and thereby the atherosclerotic process. However, the role of endothelial Cxs and their channels in atherosclerotic development after IR exposure is still poorly defined. A better understanding of the underlying biological pathways involved in secondary cardiovascular toxicity after radiotherapy would facilitate the development of effective strategies that prevent or mitigate these adverse effects. Here, we review the possible roles of intercellular Cx driven signaling and communication in radiation-induced atherosclerosis.

Radiation-Induced Salivary Gland Dysfunction: Mechanisms, Therapeutics and Future Directions

Salivary glands sustain collateral damage following radiotherapy (RT) to treat cancers of the head and neck, leading to complications, including mucositis, xerostomia and hyposalivation. Despite salivary gland-sparing techniques and modified dosing strategies, long-term hypofunction remains a significant problem. Current therapeutic interventions provide temporary symptom relief, but do not address irreversible glandular damage. In this review, we summarize the current understanding of mechanisms involved in RT-induced hyposalivation and provide a framework for future mechanistic studies. One glaring gap in published studies investigating RT-induced mechanisms of salivary gland dysfunction concerns the effect of irradiation on adjacent non-irradiated tissue via paracrine, autocrine and direct cell-cell interactions, coined the bystander effect in other models of RT-induced damage. We hypothesize that purinergic receptor signaling involving P2 nucleotide receptors may play a key role in mediating the bystander effect. We also discuss promising new therapeutic approaches to prevent salivary gland damage due to RT.

Cx43 channels and signaling via IP3/Ca2+, ATP, and ROS/NO propagate radiation-induced DNA damage to non-irradiated brain microvascular endothelial cells

Radiotherapeutic treatment consists of targeted application of radiation beams to a tumor but exposure of surrounding healthy tissue is inevitable. In the brain, ionizing radiation induces breakdown of the blood-brain barrier by effects on brain microvascular endothelial cells. Damage from directly irradiated cells can be transferred to surrounding non-exposed bystander cells, known as the radiation-induced bystander effect. We investigated involvement of connexin channels and paracrine signaling in radiation-induced bystander DNA damage in brain microvascular endothelial cells exposed to focused X-rays. Irradiation caused DNA damage in the directly exposed area, which propagated over several millimeters in the bystander area. DNA damage was significantly reduced by the connexin channel-targeting peptide Gap26 and the Cx43 hemichannel blocker TAT-Gap19. ATP release, dye uptake, and patch clamp experiments showed that hemichannels opened within 5 min post irradiation in both irradiated and bystander areas. Bystander signaling involved cellular Ca²⁺ dynamics and IP₃, ATP, ROS, and NO signaling, with Ca²⁺, IP₃, and ROS as crucial propagators of DNA damage. We conclude that bystander effects are communicated by a concerted cascade involving connexin channels, and IP₃/Ca²⁺, ATP, ROS, and NO as major contributors of regenerative signal expansion.

P2 Receptors as Therapeutic Targets in the Salivary Gland: From Physiology to Dysfunction

Although often overlooked in our daily lives, saliva performs a host of necessary physiological functions, including lubricating and protecting the oral cavity, facilitating taste sensation and digestion and maintaining tooth enamel. Therefore, salivary gland dysfunction and hyposalivation, often resulting from pathogenesis of the autoimmune disease Sjögren's syndrome or from radiotherapy of the head and neck region during cancer treatment, severely reduce the quality of life of afflicted patients and can lead to dental caries, periodontitis, digestive disorders, loss of taste and difficulty speaking. Since their initial discovery in the 1970s, P2 purinergic receptors for extracellular nucleotides, including ATP-gated ion channel P2X and G protein-coupled P2Y receptors, have been shown to mediate physiological processes in numerous tissues, including the salivary glands where P2 receptors represent a link between canonical and non-canonical saliva secretion. Additionally, extracellular nucleotides released during periods of cellular stress and inflammation act as a tissue alarmin to coordinate immunological and tissue repair responses through P2 receptor activation. Accordingly, P2 receptors have gained widespread clinical interest with agonists and antagonists either currently undergoing clinical trials or already approved for human use. Here, we review the contributions of P2 receptors to salivary gland function and describe their role in salivary gland dysfunction. We further consider their potential as therapeutic targets to promote physiological saliva flow, prevent salivary gland inflammation and enhance tissue regeneration.

P2X7 receptor deletion suppresses γ-radiation-induced hyposalivation

Head and neck cancer treatments typically involve a combination of surgery and radiotherapy, often leading to collateral damage to nearby tissues causing unwanted side effects. Radiation damage to salivary glands frequently leads to irreversible dysfunction by poorly understood mechanisms. The P2X7 receptor (P2X7R) is a ligand-gated ion channel activated by extracellular ATP released from damaged cells as "danger signals." P2X7R activation initiates apoptosis and is involved in numerous inflammatory disorders. In this study, we utilized P2X7R knockout (P2X7R^-/-) mice to determine the role of the receptor in radiation-induced salivary gland damage. Results indicate a dose-dependent increase in γ-radiation-induced ATP release from primary parotid gland cells of wild-type but not P2X7R^-/- mice. Despite these differences, apoptosis levels are similar in parotid glands of wild-type and P2X7R^-/- mice 24-72 h after radiation. However, γ-radiation caused elevated prostaglandin E₂ (PGE₂) release from primary parotid cells of wild-type but not P2X7R^-/- mice. To attempt to uncover the mechanism underlying differential PGE₂ release, we evaluated the expression and activities of cyclooxygenase and PGE synthase isoforms. There were no consistent trends in these mediators following radiation that could explain the reduction in PGE₂ release in P2X7R^-/- mice. Irradiated P2X7R^-/- mice have stimulated salivary flow rates similar to unirradiated controls, whereas irradiated wild-type mice have significantly decreased salivary flow rates compared with unirradiated controls. Notably, treatment with the P2X7R antagonist A438079 preserves stimulated salivary flow rates in wild-type mice following γ-radiation. These data suggest that P2X7R antagonism is a promising approach for preventing γ-radiation-induced hyposalivation.

Targeted and Off-Target (Bystander and Abscopal) Effects of Radiation Therapy: Redox Mechanisms and Risk/Benefit Analysis

SIGNIFICANCE

Radiation therapy (from external beams to unsealed and sealed radionuclide sources) takes advantage of the detrimental effects of the clustered production of radicals and reactive oxygen species (ROS). Research has mainly focused on the interaction of radiation with water, which is the major constituent of living beings, and with nuclear DNA, which contains the genetic information. This led to the so-called target theory according to which cells have to be hit by ionizing particles to elicit an important biological response, including cell death. In cancer therapy, the Poisson law and linear quadratic mathematical models have been used to describe the probability of hits per cell as a function of the radiation dose. Recent Advances: However, in the last 20 years, many studies have shown that radiation generates "danger" signals that propagate from irradiated to nonirradiated cells, leading to bystander and other off-target effects.

CRITICAL ISSUES

Like for targeted effects, redox mechanisms play a key role also in off-target effects through transmission of ROS and reactive nitrogen species (RNS), and also of cytokines, ATP, and extracellular DNA. Particularly, nuclear factor kappa B is essential for triggering self-sustained production of ROS and RNS, thus making the bystander response similar to inflammation. In some therapeutic cases, this phenomenon is associated with recruitment of immune cells that are involved in distant irradiation effects (called "away-from-target" i.e., abscopal effects).

FUTURE DIRECTIONS

Determining the contribution of targeted and off-target effects in the clinic is still challenging. This has important consequences not only in radiotherapy but also possibly in diagnostic procedures and in radiation protection.

TRPV4 Channel-Regulated ATP Release Contributes to γ-Irradiation-Induced Production of IL-6 and IL-8 in Epidermal Keratinocytes

External stimuli, such as radiation, induce inflammatory cytokine and chemokine production in skin, but the mechanisms involved are not completely understood. We previously showed that the P2Y11 nucleotide receptor, p38 mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-κB) all participate in interleukin (IL)-6 production induced by γ-irradiation. Here, we focused on the transient receptor potential vanilloid 4 (TRPV4) channel, which is expressed in skin keratinocytes and has been reported to play a role in inflammation. We found that irradiation of human epidermal keratinocytes HaCaT cells with 5 Gy of γ-rays (¹³⁷Cs: 0.75 Gy/min) induced IL-6 and IL-8 production. HaCaT cells treated with TRPV4 channel agonist GSK1016790A also showed increased IL-6 and IL-8 production. In both cases, IL-6/IL-8 production was not increased at 24 h after stimulation, but was increased at 48 h. ATP was released from cells exposed to γ-irradiation or TRPV4 channel agonist, and the release was suppressed by TRPV4 channel inhibitors. The γ-irradiation-induced increase in IL-6 and IL-8 production was suppressed by apyrase (ecto-nucleotidase), NF157 (selective P2Y11 receptor antagonist) and SB203580 (p38 MAPK inhibitor). GSK1016790A-induced inhibitor of kappa B-alpha (IκBα) decomposition, which causes NF-κB activation was suppressed by NF157 and SB203580, and γ-irradiation-induced IκBα decomposition was suppressed by TRPV4 channel inhibitors. Our results suggest that γ-irradiation of keratinocytes induces ATP release via activation of the TRPV4 channel, and then ATP activates P2Y11 receptor and p38 MAPK-NF-κB signaling, resulting in IL-6/IL-8 production.

The G protein-coupled P2Y₆ receptor promotes colorectal cancer tumorigenesis by inhibiting apoptosis

Colorectal tumors are immersed in an array of tumor-promoting factors including extracellular nucleotides such as uridine 5'‑diphosphate (UDP). UDP is the endogenous agonist of the G protein-coupled P2Y₆ receptor (P2Y₆R), which may contribute to the formation of a tumor-promoting microenvironment by coordinating resistance to apoptosis. Colorectal cancer (CRC) was chemically induced in P2ry6 knockout (P2ry6^-/-) mice using azoxymethane and dextran sulfate sodium challenges. Mice were euthanatized and their tumor load determined. Fixed tissues were stained for histological and immunohistochemistry analysis. Tumoroids were also prepared from CRC tumors resected from P2ry6^+/+ mice to determine the role of P2Y₆R in resistance to apoptosis, whereas HT29 carcinoma cells were used to elucidate the signaling mechanism involved in P2Y₆R anti-apoptotic effect. P2ry6^-/- mice developed a reduced number of colorectal tumors with apparent tumors having smaller volumes. Overall dysplastic score was significantly lower in P2ry6^-/- animals. Stimulation of P2Y₆R with the selective agonist MRS2693 protected HT-29 cells from TNFα-induced apoptosis. This protective effect was mediated by the stabilizing phosphorylation of the X-linked inhibitor of apoptosis protein (XIAP) by AKT. Using CRC-derived tumoroids, P2Y₆R activation was found to contribute to chemoresistance since addition of the P2Y₆R agonist MRS2693 significantly prevented the cytotoxic effect of 5-fluorouracil. The present study shows that sustained activation of P2Y₆R may contribute to intestinal tumorigenesis by blocking the apoptotic process and by contributing to chemoresistance, a substantial concern in the treatment of patients with CRC. These results suggest that P2Y₆R may represent a prime target for reducing colorectal carcinogenesis.

A sensitive method for the separation and quantification of low-level adenine nucleotides using porous graphitic carbon-based liquid chromatography and tandem mass spectrometry

A liquid chromatography coupled to heated electrospray ionization/tandem mass spectrometry (LC-HESI-MS/MS) method was developed for the simultaneous quantitative analysis of low nanomolar level adenine nucleotides AMP, ADP, ATP, cyclic AMP (cAMP), and the nucleoside adenosine. For analyte retention and separation, reverse phase chromatography using porous graphitic carbon (PGC) was employed as it provided full resolution. The erratic chromatographic behaviour characteristic of PGC, including deterioration of analyte resolution and increased peak tailing (leading to decreased sensitivity), was mitigated by incorporating acidic equilibration within runs using a quaternary gradient. Analyte resolution and chromatographic sensitivity were still lost after a period of column inactivity; hence a pre-conditioning protocol was implemented between batches to regenerate the column. These column regeneration measures also allowed elution of AMP, ADP and ATP in the sequence of mono- to tri- nucleotides, differing from conventional reverse phase elution where analytes elute with decreasing polarity. This nucleotide elution sequence has the advantage of overcoming potential mis-annotation and inaccurate quantification of smaller nucleotides caused by in-source fragmentation of ATP. The method was validated in granulosa cell conditioned media, with the LLOQs ranging between 10-50nM for most analytes. To verify the method using biological samples, nucleotide secretion was measured in granulosa cell conditioned media under various treatments known to alter their levels. Moreover, the method was applied to cumulus-oocyte complex cell lysates to examine its linearity in a complex matrix.

Role of ATP as a Key Signaling Molecule Mediating Radiation-Induced Biological Effects

Adenosine triphosphate (ATP) serves as a signaling molecule for adaptive responses to a variety of cytotoxic agents and plays an important role in mediating the radiation stress-induced responses that serve to mitigate or repair the injurious effects of γ radiation on the body. Indeed, low doses of radiation may have a net beneficial effect by activating a variety of protective mechanisms, including antitumor immune responses. On the other hand, ATP signaling may be involved in the radiation resistance of cancer cells. Here, focusing on our previous work, we review the evidence that low-dose γ irradiation (0.25-0.5 Gy) induces release of extracellular ATP, and that the released ATP mediates multiple radiation-induced responses, including increased intracellular antioxidant synthesis, cell-mediated immune responses, induction of DNA damage repair systems, and differentiation of regulatory T cells.

Single Low-Dose Radiation Induced Regulation of Keratinocyte Differentiation in Calcium-Induced HaCaT Cells

Background

We are continually exposed to low-dose radiation (LDR) in the range 0.1 Gy from natural sources, medical devices, nuclear energy plants, and other industrial sources of ionizing radiation. There are three models for the biological mechanism of LDR: the linear no-threshold model, the hormetic model, and the threshold model.

Objective

We used keratinocytes as a model system to investigate the molecular genetic effects of LDR on epidermal cell differentiation.

Methods

To identify keratinocyte differentiation, we performed western blots using a specific antibody for involucrin, which is a precursor protein of the keratinocyte cornified envelope and a marker for keratinocyte terminal differentiation. We also performed quantitative polymerase chain reaction. We examined whether LDR induces changes in involucrin messenger RNA (mRNA) and protein levels in calcium-induced keratinocyte differentiation.

Results

Exposure of HaCaT cells to LDR (0.1 Gy) induced p21 expression. p21 is a key regulator that induces growth arrest and represses stemness, which accelerates keratinocyte differentiation. We correlated involucrin expression with keratinocyte differentiation, and examined the effects of LDR on involucrin levels and keratinocyte development. LDR significantly increased involucrin mRNA and protein levels during calcium-induced keratinocyte differentiation.

Conclusion

These studies provide new evidence for the biological role of LDR, and identify the potential to utilize LDR to regulate or induce keratinocyte differentiation.

Radiosensitizing Effect of TRPV1 Channel Inhibitors in Cancer Cells

Radiosensitizers are used in cancer therapy to increase the γ-irradiation susceptibility of cancer cells, including radioresistant hypoxic cancer cells within solid tumors, so that radiotherapy can be applied at doses sufficiently low to minimize damage to adjacent normal tissues. Radiation-induced DNA damage is repaired by multiple repair systems, and therefore these systems are potential targets for radiosensitizers. We recently reported that the transient receptor potential vanilloid type 1 (TRPV1) channel is involved in early responses to DNA damage after γ-irradiation of human lung adenocarcinoma A549 cells. Therefore, we hypothesized that TRPV1 channel inhibitors would have a radiosensitizing effect by blocking repair of radiation-induced cell damage. Here, we show that pretreatment of A549 cells with the TRPV1 channel inhibitors capsazepine, AMG9810, SB366791 and BCTC suppressed the γ-ray-induced activation of early DNA damage responses, i.e., activation of the protein kinase ataxia-telangiectasia mutated (ATM) and accumulation of p53-binding protein 1 (53BP1). Further, the decrease of survival fraction at one week after γ-irradiation (2.0 Gy) was enhanced by pretreatment of cells with these inhibitors. On the other hand, inhibitor pretreatment did not affect cell viability, the number of apoptotic or necrotic cells, or DNA synthesis at 24 h after irradiation. These results suggest that inhibition of DNA repair by TRPV1 channel inhibitors in irradiated A549 cells caused gradual loss of proliferative ability, rather than acute facilitation of apoptosis or necrosis. TRPV1 channel inhibitors could be novel candidates for radiosensitizers to improve the efficacy of radiation therapy, either alone or in combination with other types of radiosensitizers.

Purinergic Signaling Is a Novel Mechanism of the Cellular Response to Ionizing Radiation

Recent studies suggest the effect of radiation is observed not only in irradiated cells but also in adjacent non-irradiated cells (bystander effect), although the mechanism has not yet been fully revealed. This bystander effect may be caused by intercellular communication via a gap junction or by messengers released from irradiated cells, such as reactive oxygen species, nitric oxide, or cytokines. However, an unknown mechanism is also possible in the bystander effect. On the other hand, it is known that extracellular ATP, ADP, uridine 5'-triphosphate (UTP), and uridine 5'-diphosphate (UDP), which are released from cells, act as intercellular signaling molecules by activating purinergic P2X and P2Y receptors (purinergic signaling). Recently, I have suggested these extracellular nucleotides may be novel mediators of a radiation-induced bystander effect, because our recent studies indicated that purinergic signaling is involved in important cellular responses to radiation. Our data indicate that ionizing irradiation causes activation of the transient receptor potential melastatin type 2 (TRPM2) channel, and then ATP is released from cells through the anion channel or connexin43 hemichannel mediated by the activation of a P2X7 receptor. The released nucleotides activate P2Y6 and P2Y12 receptors, which are involved in the DNA damage response after irradiation. Activation of the P2Y6 receptor is also involved in radiation-induced activation of the epithelial growth factor receptor-extracellular signal regulated protein kinase (EGFR-ERK)1/2 pathway and subsequent nuclear translocation of EGFR, which plays a role in DNA repair. Further, the induction of an antioxidant after irradiation is also mediated by the activation of the P2Y receptor. In conclusion, purinergic signaling could play an important role in the protective cellular response to ionizing irradiation.

ATP Release and P2 Y Receptor Signaling are Essential for Keratinocyte Galvanotaxis

Repair to damaged tissue requires directional cell migration to heal the wound. Immediately upon wounding an electrical guidance cue is created with the cathode of the electric field (EF) located at the center of the wound. Previous research has demonstrated directional migration of keratinocytes toward the cathode when an EF of physiological strength (100-150 mV/mm) is applied in vitro, but the "sensor" by which keratinocytes sense the EF remains elusive. Here we use a customized chamber design to facilitate the application of a direct current (DC) EF of physiological strength (100 mV/mm) to keratinocytes whilst pharmacologically modulating the activation of both connexin hemichannels and purinergic receptors to determine their role in EF-mediated directional keratinocyte migration, galvanotaxis. In addition, keratinocytes were exposed to DiSCAC2 (3) dye to visualize membrane potential changes within the cell upon exposure to the applied DC EF. Here we unveil ATP-medicated mechanisms that underpin the initiation of keratinocyte galvanotaxis. The application of a DC EF of 100 mV/mm releases ATP via hemichannels activating a subset of purinergic P2 Y receptors, locally, to initiate the directional migration of keratinocytes toward the cathode in vitro, the center of the wound in vivo. The delineation of the mechanisms underpinning galvanotaxis extends our understanding of this endogenous cue and will facilitate the optimization and wider use of EF devices for chronic wound treatment. J. Cell. Physiol. 230: 181-191, 2016. © 2015 Wiley Periodicals, Inc.

Autocrine Regulation of UVA-Induced IL-6 Production via Release of ATP and Activation of P2Y Receptors

Extracellular nucleotides, such as ATP, are released from cells in response to various stimuli and act as intercellular signaling molecules through activation of P2 receptors. Exposure to the ultraviolet radiation A (UVA) component of sunlight causes molecular and cellular damage, and in this study, we investigated the involvement of extracellular nucleotides and P2 receptors in the UVA-induced cellular response. Human keratinocyte-derived HaCaT cells were irradiated with a single dose of UVA (2.5 J/cm²), and ATP release and interleukin (IL)-6 production were measured. ATP was released from cells in response to UVA irradiation, and the release was blocked by pretreatment with inhibitors of gap junction hemichannels or P2X7 receptor antagonist. IL-6 production was increased after UVA irradiation, and this increase was inhibited by ecto-nucleotidase or by antagonists of P2Y11 or P2Y13 receptor. These results suggest that UVA-induced IL-6 production is mediated by release of ATP through hemichannels and P2X7 receptor, followed by activation of P2Y11 and P2Y13 receptors. Interestingly, P2Y11 and P2Y13 were associated with the same pattern of IL-6 production, though they trigger different intracellular signaling cascades: Ca²⁺-dependent and PI3K-dependent, respectively. Thus, IL-6 production in response to UVA-induced ATP release involves at least two distinct pathways, mediated by activation of P2Y11 and P2Y13 receptors.

Radiation takes its Toll

The ability to recognize and respond to universal molecular patterns on invading microorganisms allows our immune system to stay on high alert, sensing danger to our self-integrity. Our own damaged cells and tissues in pathological situations activate similar warning systems as microbes. In this way, the body is able to mount a response that is appropriate to the danger. Toll-like receptors are at the heart of this pattern recognition system that initiates innate pro-oxidant, pro-inflammatory signaling cascades and ultimately bridges recognition of danger to adaptive immunity. The acute inflammatory lesions that are formed segue into resolution of inflammation, repair and healing or, more dysfunctionally, into chronic inflammation, autoimmunity, excessive tissue damage and carcinogenesis. Redox is at the nexus of this decision making process and is the point at which ionizing radiation initially intercepts to trigger similar responses to self-damage. In this review we discuss our current understanding of how radiation-damaged cells interact with Toll-like receptors and how the immune systems interprets these radiation-induced danger signals in the context of whole-body exposures and during local tumor irradiation.

[Involvement of ATP in radiation-induced bystander effect as a signaling molecule]

We previously reported that low doses (0.25-0.5 Gy) of γ-rays induce intracellular antioxidant, radioresistant, DNA damage repair, and so on. Meanwhile, we have recently reported that ATP is released from the cells exposed to low-dose γ-rays. Here, it was investigated whether or not γ-radiation-induced release of extracellular ATP contributes to various radiation effects, in paricular, focusing on the inductions of intracellular antioxidant and DNA damage repair. Irradiation with γ-rays or exogenously added ATP increased expression of intracellular antioxidants such as thioredoxin and the increases were blocked by pretreatment with an ecto-nucleotidase in both cases. Moreover, release of ATP and autocrine/paracrine positive feedback through P2Y receptors serve to amplify the cellular repair response to radiation-induced DNA damage. To sum up, it would be suggested that ATP signaling is important for the effective induction of radiation stress response, such as protection of the body from the radiation and DNA damage repair. In addition, the possibility that this signaling is involved in the radiation resistance of cancer cells and beneficial effect on the organism of low-dose radiation and radiation adaptive response, would be further suggested.

Extracellular UDP-glucose activates P2Y14 Receptor and Induces Signal Transducer and Activator of Transcription 3 (STAT3) Tyr705 phosphorylation and binding to hyaluronan synthase 2 (HAS2) promoter, stimulating hyaluronan synthesis of keratinocytes

Hyaluronan, a major matrix molecule in epidermis, is often increased by stimuli that enhance keratinocyte proliferation and migration. We found that small amounts of UDP-sugars were released from keratinocytes and that UDP-glucose (UDP-Glc) added into keratinocyte cultures induced a specific, rapid induction of hyaluronan synthase 2 (HAS2), and an increase of hyaluronan synthesis. The up-regulation of HAS2 was associated with JAK2 and ERK1/2 activation, and specific Tyr(705) phosphorylation of transcription factor STAT3. Inhibition of JAK2, STAT3, or Gi-coupled receptors blocked the induction of HAS2 expression by UDP-Glc, the latter inhibitor suggesting that the signaling was triggered by the UDP-sugar receptor P2Y14. Chromatin immunoprecipitations demonstrated increased promoter binding of Tyr(P)(705)-STAT3 at the time of HAS2 induction. Interestingly, at the same time Ser(P)(727)-STAT3 binding to its response element regions in the HAS2 promoter was unchanged or decreased. UDP-Glc also stimulated keratinocyte migration, proliferation, and IL-8 expression, supporting a notion that UDP-Glc signals for epidermal inflammation, enhanced hyaluronan synthesis as an integral part of it.

Involvement of P2Y11 receptor in silica nanoparticles 30-induced IL-6 production by human keratinocytes

We have previously reported that P2Y11 receptor mediates IFN-γ-induced IL-6 production in human keratinocytes, suggesting the importance of purinergic signaling in skin inflammatory diseases. In this study, the involvement of various P2 receptors in IL-6 production induced by silica nanoparticle 30 (SNP30) was examined in a human keratinocyte cell line, HaCaT. Exposure to SNP30 increased IL-6 production in the cells. Ecto-nucleotidase (apyrase), a non-selective antagonist of P2Y receptors (suramin), and a selective P2Y11 receptor antagonist (NF157) all inhibited IL-6 production. Nucleotides such as ATP and UTP themselves also significantly increased IL-6 production in the cells. It was further confirmed that ATP was released from HaCaT cells exposed to SNP30. These results support the possible role of ATP in SNP30-induced IL-6 production by HaCaT cells. In conclusion, these data demonstrate that P2Y11 receptor also mediates SNP30-induced IL-6 production in human keratinocytes, confirming that the ATP-P2Y11 purinergic signaling is a common pathway of IL-6 production leading to induction of skin inflammatory diseases.

Autophagy inhibition radiosensitizes in vitro, yet reduces radioresponses in vivo due to deficient immunogenic signalling

Clinical oncology heavily relies on the use of radiotherapy, which often leads to merely transient responses that are followed by local or distant relapse. The molecular mechanisms explaining radioresistance are largely elusive. Here, we identified a dual role of autophagy in the response of cancer cells to ionizing radiation. On one hand, we observed that the depletion of essential autophagy-relevant gene products, such as ATG5 and Beclin 1, increased the sensitivity of human or mouse cancer cell lines to irradiation, both in vitro (where autophagy inhibition increased radiation-induced cell death and decreased clonogenic survival) and in vivo, after transplantation of the cell lines into immunodeficient mice (where autophagy inhibition potentiated the tumour growth-inhibitory effect of radiotherapy). On the other hand, when tumour proficient or deficient for autophagy were implanted in immunocompetent mice, it turned out that defective autophagy reduced the efficacy of radiotherapy. Indeed, radiotherapy elicited an anti-cancer immune response that was dependent on autophagy-induced ATP release from stressed or dying tumour cells and was characterized by dense lymphocyte infiltration of the tumour bed. Intratumoural injection of an ecto-ATPase inhibitor restored the immune infiltration of autophagy-deficient tumours post radiotherapy and improved the growth-inhibitory effect of ionizing irradiation. Altogether, our results reveal that beyond its cytoprotective function, autophagy confers immunogenic properties to tumours, hence amplifying the efficacy of radiotherapy in an immunocompetent context. This has far-reaching implications for the development of pharmacological radiosensitizers.

The purinergic P2Y14 receptor axis is a molecular determinant for organism survival under in utero radiation toxicity

In utero exposure of the embryo and fetus to radiation has been implicated in malformations or fetal death, and often produces lifelong health consequences such as cancers and mental retardation. Here we demonstrate that deletion of a G-protein-coupled purinergic receptor, P2Y₁₄, confers potent resistance to in utero radiation. Intriguingly, a putative P2Y₁₄ receptor ligand, UDP-glucose, phenocopies the effect of P2Y₁₄ deficiency. These data indicate that P2Y₁₄ is a receptor governing in utero tolerance to genotoxic stress that may be pharmacologically targeted to mitigate radiation toxicity in pregnancy.

Characterization of gene expression profiles at low and very low doses of ionizing radiation

The aim of the present study is to analyse by microarray techniques how gene expression is modulated after exposure to low and very low doses of ionizing radiation, to evaluate if the pattern of gene expression shows dose dependence, and to search for putative regulatory mechanisms behind the observed gene-expression modifications. For this, whole blood samples from five healthy donors were exposed in six doses between 5 and 500mGy. Total RNA extraction from CD4(+) lymphocytes was done at four different post-irradiation times. After mRNA amplification, aRNAs were hybridized on DNA microarrays. The results indicated that up-regulation was twice than down-regulation. Surprisingly, the number of modulated genes does not seem to change drastically with dose, even at the lowest dose of 5mGy. Clustering analysis revealed seven gene expression clusters with different dose dependence profiles. The functional analysis showed that the genes which increased their expression with the dose were related to p53 pathway and DNA damage response. This could be observed from 25mGy, but became very clear at doses equal or greater than 100mGy. On the other hand, genes with a constant modulation of their expression in all the tested doses were related to cellular respiration, ATP metabolic process and chromatin organization. These latter molecular mechanisms seem to be triggered at very low doses (5-25mGy). In silico promoter analysis seems to confirm the implication of transcription factors related to the pathways mentioned above.

In vivo imaging demonstrates ATP release from murine keratinocytes and its involvement in cutaneous inflammation after tape stripping

Adenosine 5'-triphosphate (ATP) release from keratinocytes has been observed in various stress models in vitro, but studies demonstrating epidermal ATP release in vivo are limited. To visualize extracellular ATP (eATP) in vivo, we developed enhanced green-emitting luciferase immobilized on agarose beads (Eluc-agarose). Subcutaneous injection of Eluc-agarose together with ATP into the dorsal skin of BALB/c mice following intraperitoneal luciferin injection produced detectable and measurable bioluminescence using an in vivo imaging system. Using Eluc-agarose, we demonstrated in vivo that bright bioluminescence was observed from 1 to 20 minutes after repeated tape stripping of murine skin. This bioluminescence was suppressed by the local administration of apyrase. Eluc-agarose bioluminescence was observed only in tape-stripped skin with transepidermal water loss (TEWL) between 100 and 140 g m(2) h(-1), indicating a loss of bioluminescence with excessive tape stripping (TEWL>140 g m(-2) h(-1)). Histologically, tape-stripped skin with detectable eATP had a viable epidermis and a subepidermal neutrophil infiltrate, and administration of apyrase reduced the inflammatory infiltrate. Neither a viable epidermis nor an upper dermal neutrophil infiltrate was observed after excessive tape stripping. These results suggest that tape stripping prompts ATP release from viable keratinocytes, which facilitates inflammatory cell migration. Eluc-agarose may be useful in the in vivo detection of eATP in murine models of skin diseases.

Involvement of P2Y₁₁ receptor in IFN-γ-induced IL-6 production in human keratinocytes

Extracellular ATP and P2 receptors are reported to be involved in interleukin-6 (IL-6) production by human keratinocytes, but the role of extracellular ATP in cytokine-induced IL-6 production remains unclear. In this study, we investigated the involvement of various P2 receptors in IL-6 production induced by the Th1 cytokine interferon-gamma (IFN-γ) in a human keratinocyte cell line, HaCaT. IFN-γ increased IL-6 production in HaCaT cells. A non-selective antagonist of P2Y receptors (suramin), a selective P2Y11 receptor antagonist (NF157), ecto-nucleotidase (apyrase), and a soluble adenylate cyclase inhibitor (KH7) all inhibited IL-6 production. It was further confirmed that ATP was released from HaCaT cells stimulated with IFN-γ. These results suggest that extracellular ATP and P2Y11 receptor are involved in IFN-γ-induced IL-6 production. Knockdown of P2Y11 receptor suppressed IL-6 production, strongly supporting this idea. In conclusion, these data demonstrate that P2Y11 receptor mediates IFN-γ-induced IL-6 production in human keratinocytes, and suggest the importance of purinergic signaling in IFN-γ-induced skin inflammatory conditions, such as psoriasis.

Susceptibility to bystander DNA damage is influenced by replication and transcriptional activity

Direct cellular DNA damage may lead to genome destabilization in unexposed, bystander, cells sharing the same milieu with directly damaged cells by means of the bystander effect. One proposed mechanism involves double strand break (DSB) formation in S phase cells at sites of single strand lesions in the DNA of replication complexes, which has a more open structure compared with neighboring DNA. The DNA in transcription complexes also has a more open structure, and hence may be susceptible to bystander DSB formation from single strand lesions. To examine whether transcription predisposes non-replicating cells to bystander effect-induced DNA DSBs, we examined two types of primary cells that exhibit high levels of transcription in the absence of replication, rat neurons and human lymphocytes. We found that non-replicating bystander cells with high transcription rates exhibited substantial levels of DNA DSBs, as monitored by γ-H2AX foci formation. Additionally, as reported in proliferating cells, TGF-β and NO were found to mimic bystander effects in cell populations lacking DNA synthesis. These results indicate that cell vulnerability to bystander DSB damage may result from transcription as well as replication. The findings offer insights into which tissues may be vulnerable to bystander genomic destabilization in vivo.

Involvement of connexin43 hemichannel in ATP release after γ-irradiation

Ionizing radiation induces biological effects not only in irradiated cells but also in non-irradiated cells, which is called the bystander effect. Recently, in vivo and in vitro experiments have suggested that both gap junction hemichannel connexin43 (Cx43) and extracellular adenosine triphosphate (ATP) released from cells play a role in the bystander effect. We have reported that γ-irradiation induces ATP release from B16 melanoma cells, which is dependent on the P2X(7) receptor. However, the mechanism of ATP release caused by irradiation remains unclear. We here show the involvement of Cx43 in P2X(7) receptor-dependent ATP release after 0.5 Gy γ-irradiation. Inhibitors of gap junction hemichannels and an inhibitory peptide for Cx43 (gap26), but not an inhibitory peptide for pannexin1 (Panx1), significantly blocked γ-irradiation-induced ATP release from B16 melanoma cells. We confirmed high expression of Cx43 mRNA in B16 melanoma cells. These results suggest involvement of Cx43 in radiation-induced ATP release. We found that after 0.5 Gy γ-irradiation tyrosine phosphorylation was significantly blocked by P2X(7) receptor antagonist, but not gap26, suggesting that tyrosine phosphorylation is a downstream event from the P2X(7) receptor. Since tyrosine kinase inhibitor significantly suppressed radiation-induced ATP release, tyrosine phosphorylation appears to play an important role in the Cx43-mediated ATP release downstream of the P2X(7) receptor. In conclusion, the Cx43 hemichannel, which lies downstream of the P2X(7) receptor, is involved in ATP release in response to radiation. Our results suggest a novel mechanism for radiation-induced biological effects mediated by both ATP and Cx43.

Autocrine regulation of γ-irradiation-induced DNA damage response via extracellular nucleotides-mediated activation of P2Y6 and P2Y12 receptors

A key component of the response to DNA damage caused by ionizing radiation is DNA repair. Release of extracellular nucleotides, such as ATP, from cells plays a role in signaling via P2 receptors. We show here that release of ATP, followed by activation of P2Y receptors, is involved in the response to γ-irradiation-induced DNA damage. Formation of phosphorylated histone variant H2AX (γH2AX) foci, which are induced in nuclei by DNA damage and contribute to accumulation of DNA-repair factors, was increased at 1-3h after γ-ray irradiation (2.0Gy) of human lung cancer A549 cells. Focus formation was suppressed by pre-treatment with the ecto-nucleotidase apyrase. Pre-treatment with ecto-nucleotidase inhibitor ARL67156 or post-treatment with ATP or UTP facilitated induction of γH2AX, indicating that extracellular nucleotides play a role in induction of γH2AX foci. Next, we examined the effect of P2 receptor inhibitors on activation of ataxia telangiectasia mutated (ATM; a protein kinase) and accumulation of 53BP1 (a DNA repair factor), both of which are important for DNA repair, at DNA damage sites. P2Y6 receptor antagonist MRS2578, P2Y12 receptor antagonist clopidogrel, and P2X7 receptor antagonists A438079 and oxATP significantly inhibited these processes. Release of ATP was detected within 2.5min after irradiation, but was blocked by A438079. Activation of ATM and accumulation of 53BP1 were decreased in P2Y6 or P2Y12 receptor-knockdown cells. We conclude that autocrine/paracrine signaling through P2X7-dependent ATP release and activation of P2Y6 and P2Y12 receptors serves to amplify the cellular response to DNA damage caused by γ-irradiation.

Involvement of P2Y receptors in the protective effect of ATP towards the cell damage in HaCaT cells exposed to H₂O₂

It has recently been reported that activation of P2Y(1) receptor, one of the purine receptors, by extracellular nucleotides induces cytoprotection against oxidative stress. In this study, we examined the protective effect of ATP on the cell damage in human epidermal keratinocyte HaCaT cells exposed to H(2)O(2) via the P2Y receptor-mediated induction of intracellular antioxidants. The cells were damaged by exposure to H(2)O(2) in a dose- and time-dependent manner. The damage induced by 7.5 mM H(2)O(2) was blocked by pretreatment of the cells with ATP (1-10 µM). The protective effect of ATP was significantly reduced by P2Y receptor antagonists. Exogenously added ATP induced various intracellular antioxidants, including thiol-containing proteins, Cu/Zn superoxide dismutase (SOD) and thioredoxin-1, in HaCaT cells. In conclusion, it was found that ATP protected the cells from the H(2)O(2)-induced cell damages via the P2Y receptor-mediated induction of intracellular antioxidants.

Induction of extracellular ATP mediates increase in intracellular thioredoxin in RAW264.7 cells exposed to low-dose γ-rays

We previously showed that low doses (0.25-0.5 Gy) of γ-rays elevated thioredoxin (Trx-1) in various organs of mice after whole-body irradiation. Also, it is reported that extracellular ATP, which is released in response to various stresses, regulates the expression of intracellular antioxidants through activation of P2 receptors. We have recently found that low-dose γ-rays induce ATP release from the exposed cells. However, it is not yet clear whether the radiation-induced extracellular ATP modulates the cellular redox balance. Here, we investigated whether γ-ray irradiation-induced release of extracellular ATP contributes to the induction of the cellular antioxidant Trx-1, using mouse macrophage-like RAW264.7 cells. Irradiation with γ-rays or exogenously added ATP increased the expression of Trx-1, and in both cases the increase was blocked by pretreatment with an ectonucleotidase, apyrase. Then, the involvement of ATP-dependent reactive oxygen species (ROS) generation in the increase in antioxidant capacity was examined. ATP stimulation promoted the generation of intracellular ROS and also increased Trx-1 expression. The increase in Trx-1 expression was significantly suppressed by pretreatment of the cells with antioxidants. In conclusion, the γ-ray irradiation-induced release of extracellular ATP may, at least in part, contribute to the production of ROS via purinergic signaling, leading to promotion of intracellular antioxidants as an adaptive response to an oxidative stress.

Interaction between ionizing radiation and estrogen: what we are missing?

Following complexity as a new approach in science of 21st century biomonitoring of biological effects caused by ionizing radiation received an option of a new dimension. Insight in biological response of mammals to ionizing radiation exposure by integration of genome, non-genome and distant organ bystander effects will significantly change evaluation of health risk and preventive measures. Impact of estrogen on carcinogenesis caused by occupational or accidental exposure to ionizing radiation additionally enables biodosimetry to recognize vulnerable subpopulations according to gender and age. Estrogen, as a potent molecule involved in number of biological pathways during development and adulthood, shows close interaction with pathological processes launched by overexposure to ionizing radiation which should be included in future research and radiation protection.

Role of connexin43 and ATP in long-range bystander radiation damage and oncogenesis in vivo

Ionizing radiation is a genotoxic agent and human carcinogen. Recent work has questioned long-held dogmas by showing that cancer-associated genetic alterations occur in cells and tissues not directly exposed to radiation, questioning the robustness of the current system of radiation risk assessment. In vitro, diverse mechanisms involving secreted soluble factors, gap junction intercellular communication (GJIC) and oxidative metabolism are proposed to mediate these indirect effects. In vivo, the mechanisms behind long-range 'bystander' responses remain largely unknown. Here, we investigate the role of GJIC in propagating radiation stress signals in vivo, and in mediating radiation-associated bystander tumorigenesis in mouse central nervous system using a mouse model in which intercellular communication is downregulated by targeted deletion of the connexin43 (Cx43) gene. We show that GJIC is critical for transmission of oncogenic radiation damage to the non-targeted cerebellum, and that a mechanism involving adenosine triphosphate release and upregulation of Cx43, the major GJIC constituent, regulates transduction of oncogenic damage to unirradiated tissues in vivo. Our data provide a novel hypothesis for transduction of distant bystander effects and suggest that the highly branched nervous system, similar to the vascular network, has an important role.

Involvement of P2Y6 receptor in p38 MAPK-mediated COX-2 expression in response to UVB irradiation of human keratinocytes

Ultraviolet B (UVB) radiation induces inflammation in human skin. Extracellular nucleotides are released from cells in response to various stimuli and act as intercellular signaling molecules through activation of P2 receptors. In this study, we investigated the involvement of extracellular nucleotides and P2 receptors in UVB-radiation-induced inflammation using human keratinocyte-derived HaCaT cells. UVB radiation induced rapid ATP release from HaCaT cells; this was inhibited by pretreatment with anion transporter blockers or maxi-anion channel blockers. In addition, the radiation-induced activation of p38 MAPK was significantly blocked by pretreatment with ecto-nucleotidase (apyrase) or P2Y6 receptor antagonist (MRS2578). Expression of COX-2, mediated by activation of p38 MAPK, was also induced by UVB radiation. Both pretreatment with MRS2578 and knockdown of the P2Y6 receptor by siRNA transfection attenuated the induction of COX-2 in HaCaT cells exposed to UVB radiation. Our results indicate that UVB radiation evokes ATP release from human keratinocytes and also that activation of P2Y6 receptor mediates the UVB-radiation-induced activation of p38 MAPK and expression of COX-2. Thus P2Y6 receptor is a mediator of UVB-radiation-induced inflammatory responses in keratinocytes.

P2Y6 receptors and ADAM17 mediate low-dose gamma-ray-induced focus formation (activation) of EGF receptor

The EGF receptor (EGFR) is frequently expressed in tumors of epithelial origin. Although it is well known that ionizing radiation induces activation of EGFR, the mechanism remains unknown. Recently, we reported that activation of P2Y receptors is involved in γ-radiation-induced activation of extracellular signal-regulated kinase1/2 (ERK1/2), which is dependent on activation of EGFR. Here we focused on the mechanism of activation of EGFR in response to low-dose γ radiation, mainly in terms of the activation-associated formation of EGFR foci in A549 cells. Irradiation of cells with 0.1 Gy γ rays induced biphasic phosphorylation of EGFR and ERK1/2 as well as biphasic formation of EGFR foci. The radiation-induced focus formation of EGFR was abolished by ecto-nucleotidase, P2Y receptor antagonists and knockdown of P2Y6 receptor, suggesting the involvement of extracellular nucleotides and activation of P2Y6 receptors in this process. Further, a disintegrin and metalloprotease 17 (ADAM17) is expressed in A549 cells and an ADAM17 inhibitor significantly blocked the radiation-induced focus formation of EGFR. We conclude that activation of both P2Y6 receptors and ADAM17 mediates the low-dose γ-radiation-induced activation of EGFR, as evaluated in terms of focus formation, in A549 cells.