The Role of TGF-β3 in Radiation Response

Transforming growth factor-beta 3 (TGF-β3) is a ubiquitously expressed multifunctional cytokine involved in a range of physiological and pathological conditions, including embryogenesis, cell cycle regulation, immunoregulation, and fibrogenesis. The cytotoxic effects of ionizing radiation are employed in cancer radiotherapy, but its actions also influence cellular signaling pathways, including that of TGF-β3. Furthermore, the cell cycle regulating and anti-fibrotic effects of TGF-β3 have identified it as a potential mitigator of radiation- and chemotherapy-induced toxicity in healthy tissue. This review discusses the radiobiology of TGF-β3, its induction in tissue by ionizing radiation, and its potential radioprotective and anti-fibrotic effects.

Low-Dose-Rate Radiation-Induced Secretion of TGF-β3 Together with an Activator in Small Extracellular Vesicles Modifies Low-Dose Hyper-Radiosensitivity through ALK1 Binding

Hyper-radiosensitivity (HRS) is the increased sensitivity to low doses of ionizing radiation observed in most cell lines. We previously demonstrated that HRS is permanently abolished in cells irradiated at a low dose rate (LDR), in a mechanism dependent on transforming growth factor β3 (TGF-β3). In this study, we aimed to elucidate the activation and receptor binding of TGF-β3 in this mechanism. T-47D cells were pretreated with inhibitors of potential receptors and activators of TGF-β3, along with addition of small extracellular vesicles (sEVs) from LDR primed cells, before their radiosensitivity was assessed by the clonogenic assay. The protein content of sEVs from LDR primed cells was analyzed with mass spectrometry. Our results show that sEVs contain TGF-β3 regardless of priming status, but only sEVs from LDR primed cells remove HRS in reporter cells. Inhibition of the matrix metalloproteinase (MMP) family prevents removal of HRS, suggesting an MMP-dependent activation of TGF-β3 in the LDR primed cells. We demonstrate a functional interaction between TGF-β3 and activin receptor like kinase 1 (ALK1) by showing that TGF-β3 removes HRS through ALK1 binding, independent of ALK5 and TGF-βRII. These results are an important contribution to a more comprehensive understanding of the mechanism behind TGF-β3 mediated removal of HRS.

Integrated Modelling of Cell Responses after Irradiation for DNA-Targeted Effects and Non-Targeted Effects

Intercellular communication after ionizing radiation exposure, so-called non-targeted effects (NTEs), reduces cell survival. Here we describe an integrated cell-killing model considering NTEs and DNA damage along radiation particle tracks, known as DNA-targeted effects (TEs) based on repair kinetics of DNA damage. The proposed model was applied to a series of experimental data, i.e., signal concentration, DNA damage kinetics, cell survival curve and medium transfer bystander effects (MTBEs). To reproduce the experimental data, the model considers the following assumptions: (i) the linear-quadratic (LQ) function as absorbed dose to express the hit probability to emit cell-killing signals, (ii) the potentially repair of DNA lesions induced by NTEs, and (iii) lower efficiency of repair for the damage in NTEs than that in TEs. By comparing the model results with experimental data, we found that signal-induced DNA damage and lower repair efficiency in non-hit cells are responsible for NTE-related repair kinetics of DNA damage, cell survival curve with low-dose hyper-radiosensitivity (HRS) and MTBEs. From the standpoint of modelling, the integrated cell-killing model with the LQ relation and a different repair function for NTEs provide a reasonable signal-emission probability and a new estimation of low-dose HRS linked to DNA repair efficiency.

TGF-B3 Dependent Modification of Radiosensitivity in Reporter Cells Exposed to Serum From Whole-Body Low Dose-Rate Irradiated Mice

Prior findings in vitro of a TGF-β3 dependent mechanism induced by low dose-rate irradiation and resulting in increased radioresistance and removal of low dose hyper-radiosensitivity (HRS) was tested in an in vivo model. DBA/2 mice were given whole-body irradiation for 1 h at low dose-rates (LDR) of 0.3 or 0.03 Gy/h. Serum was harvested and added to RPMI (4% mouse serum and 6% bovine serum).This medium was transferred to reporter cells (T-47D breast cancer cells or T98G glioblastoma cells). The response to subsequent challenge irradiation of the reporter cells was measured by the colony assay. While serum from unirradiated control mice had no effect on the radiosensitivity in the reporter cells, serum from mice given 0.3 Gy/h or 0.03 Gy/h for 1 h removed HRS and also increased survival in response to doses up to 5 Gy. The effect lasted for at least 15 months after irradiation. TGF-β3 neutralizer added to the medium containing mouse serum inhibited the effect. Serum from mice given irradiation of 0.3 Gy/h for 1 h and subsequently treated with iNOS inhibitor 1400W did not affect radiosensitivity in reporter cells; neither did serum from the unirradiated progeny of mice given 1h LDR whole-body irradiation.

The role of interleukin-13 in the removal of hyper-radiosensitivity by priming irradiation

It has previously been demonstrated that the presence of fetal bovine serum is necessary for TGF-β3 (transforming growth factor beta 3)-dependent elimination of low-dose hyper-radiosensitivity (HRS) in cells by 1 h of low-dose-rate γ-irradiation (0.2-0.3 Gy/h). The purpose of the present study was to identify the serum constituent involved. Two human HRS-positive (T-47D, T98G) cell lines were used. The effects of different pretreatments on HRS were investigated using the colony assay. Total inducible nitric oxide synthase (iNOS) levels were measured using a cell-based ELISA assay. The serum factor was identified as interleukin-13 (IL-13). In order for low dose-rate irradiation to eliminate HRS through the TGF-β3-dependent mechanism, the cells must be exposed to IL-13 first. Inhibiting receptor IL-13Rα2 showed that this receptor is involved in the response. Adding IL-13 to serum-free medium restored the properties of full medium but not when an inhibitor of proprotein convertase activity was added together with IL-13. The presence of IL-13 resulted in upregulation of total iNOS protein levels. Thus, this study indicates that IL-13 interacts with the cells though receptor IL-13Rα2 and induces upregulation of iNOS and activation of one or more furin-like proprotein convertases.

The roles of TGF-β3 and peroxynitrite in removal of hyper-radiosensitivity by priming irradiation

PURPOSE

To investigate the mechanisms inducing and maintaining the permanent elimination of low dose hyper-radiosensitivity (HRS) in cells given a dose of 0.3 Gy at low dose-rate (LDR) (0.3 Gy/h).

MATERIALS AND METHODS

Two human HRS-positive cell lines (T-47D, T98G) were used. The effects of pretreatments with transforming growth factor beta (TGF-β) neutralizers, TGF-β3 or peroxynitrite scavenger on HRS were investigated using the colony assay. Cytoplasmic levels of TGF-β3 were measured using post-embedding immunogold electron microscopic analysis.

RESULTS

TGF-β3 neutralizer inhibited the removal of HRS by LDR irradiation. Adding 0.001 ng/ml TGF-β3 to cells removed HRS in T98G cells while 0.01 ng/ml additionally induced resistance to higher doses. Cytoplasmic levels of TGF-β3 were higher in LDR-primed cells than in unirradiated cells. The presence of the peroxynitrite scavenger uric acid inhibited the effect of LDR irradiation. Furthermore, the permanent elimination of HRS in LDR-primed cells was reversed by treatment with uric acid. The removal of HRS by medium from hypoxic cells was inhibited by adding TGF-β3 neutralizer to the medium before transfer or by adding hypoxia inducible factor 1 (HIF-1) inhibitor chetomin to the cell medium during hypoxia.

CONCLUSIONS

TGF-β3 is involved in the regulation of cellular responses to small doses of acute irradiation. TGF-β3 activation seems to be induced by low dose-rate irradiation by a mechanism involving inducible nitric oxide (iNOS) and peroxynitrite, or during cycling hypoxia by a mechanism most likely involving HIF-1. The study suggests methods to turn resistance to doses in the HRS-range on (by TGF-β3) or off (by TGF-β3 neutralizer or by peroxynitrite inhibition).

The role of nitric oxide radicals in removal of hyper-radiosensitivity by priming irradiation

In this study, a mechanism in which low-dose hyper-radiosensitivity (HRS) is permanently removed, induced by low-dose-rate (LDR) (0.2-0.3 Gy/h for 1 h) but not by high-dose-rate priming (0.3 Gy at 40 Gy/h) was investigated. One HRS-negative cell line (NHIK 3025) and two HRS-positive cell lines (T-47D, T98G) were used. The effects of different pretreatments on HRS were investigated using the colony assay. Cell-based ELISA was used to measure nitric oxide synthase (NOS) levels, and microarray analysis to compare gene expression in primed and unprimed cells. The data show how permanent removal of HRS, previously found to be induced by LDR priming irradiation, can also be induced by addition of nitric oxide (NO)-donor DEANO combined with either high-dose-rate priming or exposure to prolonged cycling hypoxia followed by reoxygenation, a treatment not involving radiation. The removal of HRS appears not to involve DNA damage induced during priming irradiation as it was also induced by LDR irradiation of cell-conditioned medium without cells present. The permanent removal of HRS in LDR-primed cells was reversed by treatment with inducible nitric oxide synthase (iNOS) inhibitor 1400W. Furthermore, 1400W could also induce HRS in an HRS-negative cell line. The data suggest that LDR irradiation for 1 h, but not 15 min, activates iNOS, and also that sustained iNOS activation is necessary for the permanent removal of HRS by LDR priming. The data indicate that nitric oxide production is involved in the regulatory processes determining cellular responses to low-dose-rate irradiation.