A Proteomic Study Suggests Stress Granules as New Potential Actors in Radiation-Induced Bystander Effects

Differential Formation of Stress Granules in Radiosensitive and Radioresistant Head and Neck Squamous Cell Carcinoma Cells

PURPOSE

Stress granules (SGs) are cytoplasmic aggregates in which mRNAs and specific proteins are trapped in response to a variety of damaging agents. They participate in the cellular defense mechanisms. Currently, their mechanism of formation in response to ionizing radiation and their role in tumor-cell radiosensitivity remain elusive.

METHODS AND MATERIALS

The kinetics of SG formation was investigated after the delivery of photon irradiation at different doses to head and neck squamous cell carcinoma cell lines with different radiosensitivities and the HeLa cervical cancer cell line (used as reference). In parallel, the response to a canonical inducer of SGs, sodium arsenite, was also studied. Immunolabeling of SG-specific proteins and mRNA fluorescence in situ hybridization enabled SG detection and quantification. Furthermore, a ribopuromycylation assay was used to assess the cell translational status. To determine whether reactive oxygen species were involved in SG formation, their scavenging or production was induced by pharmacologic pretreatment in both SCC61 and SQ20B cells.

RESULTS

Photon irradiation at different doses led to the formation of cytoplasmic foci that were positive for different SG markers. The presence of SGs gradually increased from 30 minutes to 2 hours postexposure in HeLa, SCC61, and Cal60 radiosensitive cells. In turn, the SQ20B and FaDu radioresistant cells did not form SGs. These results indicated a correlation between sensitivity to photon irradiation and SG formation. Moreover, SG formation was significantly reduced by reactive oxygen species scavenging using dimethyl sulfoxide in SCC61 cells, which supported their role in SG formation. However, a reciprocal experiment in SQ20B cells that depleted glutathione using buthionine sulfoximide did not restore SG formation in these cells.

CONCLUSIONS

SGs are formed in response to irradiation in radiosensitive, but not in radioresistant, head and neck squamous cell carcinoma cells. Interestingly, compared with sodium arsenite-induced SGs, photon-induced SGs exhibited a different morphology and cellular localization. Moreover, photon-induced SGs were not associated with the inhibition of translation; rather, they depended on oxidative stress.

Role of stress granules in tumorigenesis and cancer therapy

Stress granules (SGs) are membrane-less organelles that cell forms via liquid-liquid phase separation (LLPS) under stress conditions such as oxidative stress, ER stress, heat shock and hypoxia. SG assembly is a stress-responsive mechanism by regulating gene expression and cellular signaling pathways. Cancer cells face various stress conditions in tumor microenvironment during tumorigenesis, while SGs contribute to hallmarks of cancer including proliferation, invasion, migration, avoiding apoptosis, metabolism reprogramming and immune evasion. Here, we review the connection between SGs and cancer development, the limitation of SGs on current cancer therapy and promising cancer therapeutic strategies targeting SGs in the future.

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.

Low dose radiation mechanisms: The certainty of uncertainty

This paper reviews the current understanding of low dose radiobiology, and how it has evolved from classical target theory. It highlights the uncertainty around low dose effects, which is due in part to the complexity of "context" surrounding the ultimate expression of biological effects following low dose exposure. The paper makes special reference to low dose non-targeted effects which, are currently ignored in radiation protection and population level risk assessment, because it is unclear what they mean for risk. The view of the authors is that this "lack of clarity" about what the effects mean is precisely the point. It indicates the uncertainty of outcomes after a given exposure. The uncertainty stems from multiple outcome options resulting from the intrinsic uncertainty of the stochastic interaction of low dose radiation with matter. This uncertainty should be embraced rather than eschewed. The impacts of the uncertainties identified in this paper is explored and an approach to quantifying mutation probability in relation to dose is presented.

Histological and impedance changes of skeletal muscle by whole-body critical irradiation in a rat model

In this study, the electrical resistance of the whole body and histological changes of skeletal muscle were investigated in rats according to the increase in radiation dose. A total of 15 male Sprague-Dawley rats (5-weeks-old) were randomly divided into 5 groups (each, n = 3). Each group received 1 Gy, 5 Gy, 10 Gy and 20 Gy systemic exposure, and the non-irradiated group was used as a control for morphological comparison. After attaching an electrode clip to the forelimb of the rat, an AC frequency was applied before and 4 days after irradiation using an impedance/gain-phase analyzer, and the measurement system was automatically controlled with LabVIEW. Comparing to before irradiation after 4 days, the difference in the average impedance values at 1 Gy, 5 Gy, 10 Gy, and 20 Gy was 1188±989 ohm, 3076±2251 ohm, 7650±6836 ohm, and 10478±6250 ohm, respectively. By comparing the normal group and the experimental group, muscle fiber atrophy and collagen fibers around blood vessels were observed (p < 0.05, control group vs 5 Gy or more high-dose group). These results confirmed the previously reported morphological changes of skeletal muscle and our hypothesis that whole-body impedance measurement enables to reflect tissue changes after irradiation.