The cross-talk between Bax, Bcl2, caspases, and DNA damage in bystander HepG2 cells is regulated by γ-radiation dose and time of conditioned media transfer.

Although radiation-induced bystander effects have been broadly explored in various biological systems, the molecular mechanisms and the consequences of different regulatory factors (dose, time, cell type) on bystander responses are not clearly understood. This study investigates the effects of irradiated cell-conditioned media (ICCM) collected at different times post-irradiation on bystander cancer cells regarding DNA damage and apoptosis induction. Human hepatocellular carcinoma HepG2 cells were exposed to γ-ray doses of 2 Gy, 5 Gy, and 8 Gy. In the early and late stages (1 h, 2 h, and 24 h) after irradiation, the ICCM was collected and transferred to unirradiated cells. Compared to control, bystander cells showed an increased level of H2AX phosphorylation, mitochondrial membrane depolarization, and elevation of intrinsic apoptotic pathway mediators such as p53, Bax, cas9, cas-3, and PARP cleavage. These results were confirmed by phosphatidylserine (PS) externalization and scanning electron microscopic observations, suggesting a rise in bystander HepG2 cell apoptosis. Anti-apoptotic Bcl2-level and viability were lower in bystander cells compared to control. The highest effects were observed in 8 Gy γ radiation-induced bystander cells. Even though the bystander effect was persistent at all time points of the study, ICCM at the early time points (1 or 2 h) had the most significant impact on the apoptosis markers in bystander cells. Nevertheless, 24 h ICCM induced the highest increase in H2AX and p53 phosphorylation and Bax levels. The effects of ICCM of irradiated HepG2 cells were additionally studied in normal liver cells BRL-3A to simulate actual radiotherapy conditions. The outcomes suggest that the expression of the signaling mediators in bystander cells is highly dynamic. A cross-talk between those signaling mediators regulates bystander responses depending on the radiation dose and time of incubation post-irradiation.

The interaction of oxidative stress with MAPK, PI3/AKT, NF-κB, and DNA damage kinases influences the fate of γ-radiation-induced bystander cells.

Oxidative stress is associated with the induction of a plethora of effects on cellular macromolecules and signaling cascades. The onset of oxidative imbalance characterizes irradiated cells. The present study investigates the effects of ionizing radiation on oxidative stress induction in bystander cells and their interactions with critical cell signaling mediators. The effect of irradiated cell-conditioned medium (ICCM) from γ-irradiated hepatocellular carcinoma (HepG2) cells were studied in bystander HepG2 and normal liver (BRL-3A) cells at early (1 h, 2 h) and later (24 h) time points post-irradiation. Although ROS generation and lipid peroxidation showed the highest effects in both bystander cell groups at the early time points, antioxidant enzymes superoxide dismutase and catalase showed the lowest activity. Oxidative stress was persistent up to 24 h, but the highest level was seen in 1 h ICCM treated 8By cells. Although the levels of all pro-survival signaling factors (p-PI3K, p-Akt, p-p38MAPK, p-JNK, and p-NFκB) increased in bystander HepG2 cells, they showed a significant decrease in bystander BRL-3A cells. JAK2-STAT3 activation, however, was reduced only in BRL-3A cells, with no effect in HepG2 cells. However, in both bystander cell groups, activation of DNA damage sensors ATM, ATR, and cell cycle inhibitor p21 increased. Elevated ROS levels down-regulated the activation of PI3K, Akt, JNK, and NF-κB in BRL-3A cells but enhanced the activation of ATM and p21. In contrast, in HepG2 cells, increased ROS level elevated the activation of PI3K, JNK, p38MAPK, NF-κB with no effect on p-ATM or p21. ROS differentially influenced the interactions between the signaling mediators in the bystander cells. p-ATR levels, although increased in both bystander cell groups, showed no association with other factors. ICCM from the same HepG2 cells differently affected signaling factors in two groups of cells, highlighting the critical significance of the study in the field of radiation biology.

External modulators and redox homeostasis: Scenario in radiation-induced bystander cells.

Redox homeostasis is imperative to maintain normal physiologic and metabolic functions. Radiotherapy disturbs this balance and induces genomic instability in diseased cells. However, radiation-induced effects propagate beyond the targeted cells, affecting the adjacent non-targeted cells (bystander effects). The cellular impact of radiation, thus, encompasses both targeted and non-targeted effects. Use of external modulators along with radiation can increase radio-therapeutic efficiency. The modulators' classification as protectors or sensitizers depends on interactions with damaged DNA molecules. Thus, it is necessary to realize the functions of various radio-sensitizers or radio-protectors in both irradiated and bystander cells. This review focuses on some modulators of radiation-induced bystander effects (RIBE) and their action mechanisms. Knowledge about the underlying signaling cross-talk may promote selective sensitization of radiation-targeted cells and protection of bystander cells.

Radiation-induced bystander phenomenon: insight and implications in radiotherapy.

Despite the extensive utilization of radiotherapy in cancer treatments, clinicians often face challenges in achieving desired outcomes. This is because of the existence of secondary radiation effects like bystander phenomenon that influence radiation nontargeted cells and tissues. The concept of bystander effects of radiation was first proposed in the latter half of the last century which led to a paradigm shift in classical dogma of radiation biology. This review primarily focuses on the diverse mechanisms of bystander signal production and response under the influence of different sources of radiation like α-, ß-, γ rays as well as protons and ions. The dependency of bystander effects on cell type and linear energies are also reviewed. Furthermore, the reciprocal effects exerted by bystander cells on radiation-targeted cells have been discussed. Radiation-induced signaling in targeted cells stimulates the release of different mediators that induce a plethora of molecular effects on adjacent unirradiated cells, leading to cell death or proliferation. These responses are regulated by the cell type and linear energies of radiation. Different radiation sources have diverse damaging impacts on bystander cells wherein the complex interplay of signaling molecules, inflammation, apoptosis, necrosis, and autophagy determine the fate of the cells. Further, the feedback mechanism of bystander cells controls the severity of effects in radiation-targeted cells. From the existing literature, it is difficult to speculate how far the fundamental mechanisms of induction of bystander effects by irradiated cancer cells differ from that of normal cells, although cell-specific responses are observed in bystander cells. To have an accurate idea about the actual scenario, consideration of all possible variations of experimental conditions is necessary. This is crucial for optimal utilization of radiotherapy protocols to serve twofold purposes, increase radiotherapy efficiency while causing minimal health risks.