Nina Jeppesen Edin

Low dose hyper-radiosensitivity is eliminated during exposure to cycling hypoxia but returns after reoxygenation

Purpose: To investigate the effect of cycling hypoxia on low dose hyper-radiosensitivity (HRS). Materials and methods: Human breast tumor T-47D cells were grown in a hypoxia workstation operated at 4% O2 for 3–6 weeks and the pericellular oxygen concentration was recorded every 20 minutes. The presence of HRS in response to subsequent challenge irradiation was measured by clonogenic survival. Results: T-47D cells adapted to growing with 4% O2 in the gas phase but showed no HRS. However, HRS was recovered after between 48 h and two weeks of reoxygenation at 20% O2. Medium transferred from the hypoxic T-47D cells removed HRS in recipient cells grown in ambient air. Cells irradiated with X-rays showed a shallower HRS-‘dip’ and a lower dc-value (dose where the change from the hypersensitive to the induced repair response is 63% complete) compared to cells irradiated with 60Co γ-rays. Conclusions: Cycling hypoxia transiently eliminates HRS in T-47D cells in vitro. This may partly explain the diverging results of in vivo studies of HRS. The effect of cycling hypoxia on HRS is comparable to our previous findings for T-47D cells receiving medium transfer from cells irradiated with 0.3 Gy at 0.3 Gy/h.

Response of chronic hypoxic cells to low dose-rate irradiation.

Purpose: Compare the sensitivity of human cells in vitro to low dose-rate irradiation in air and in moderate hypoxia (4% O2). Materials and methods: Continuous low dose-rate β-irradiation at a dose rate of 0.015 or 0.062 Gy/h was given to human T-47D breast cancer cells by incorporation of [3H] -labelled valine into cellular protein. Acute irradiation at a dose rate of 0.4 Gy/min was performed using [137Cs]γ-irradiation. Cells were cultivated in an atmosphere with 4% O2 using an INVIVO2 hypoxia cabinet. Results: When grown in ambient air with continuous irradiation, T-47D cells were able to continue growth for at least 23 weeks at a dose-rate of 0.015 Gy/h with a surviving fraction stabilized at around 60%. When the dose rate was increased to 0.062 Gy/h the cell culture died out after about 23 days (corresponding to about 22 Gy). When grown in an atmosphere with 4% O2 we surprisingly found that the continuously irradiated T-47D cells (0.015 Gy/h) were severely inhibited in their growth, and cell death became extensive after about 3 weeks while un-irradiated cells continued growth seemingly unaffected by this low oxygenation. Peri cellular oxygenation varied between 4% and below 0.1% over an ordinary passage due to diffusion-limitations through the 2 mm deep medium. Online O2-recordings over a whole passage showed that oxygen was more depleted in the irradiated compared to the un-irradiated cultures indicating increased respiration during irradiation. While cells growing attached to the bottom were inhibited and inactivated during irradiation it was found that cells attached high up in the neck region, i.e., having only a shallow layer of medium above them, survived and formed colonies. When cells cultivated in 4% O2 for 7 weeks were irradiated with acute doses of 137Cs γ-rays, the radiosensitivity was the same as for cells cultivated in ambient air. Conclusion: Continuous irradiation with 0.015 Gy/h for several weeks results in a stronger inhibition for T-47D cells grown in an atmosphere with 4% as compared to 20% O2. The data indicate that this may be due to increased oxygen consumption resulting in more severe hypoxia in [3H]-incorporating compared to control (un-irradiated) cells.

Recovery of low-dose hyper-radiosensitivity following a small priming dose depends on priming dose-rate

T-47D human breast cancer cells were irradiated with 60Co γ-radiation and radiation response was measured by loss of ability of single-cells to form colonies. The influence of the dose-rate on the ability of priming irradiation to abolish Low-Dose Hyper-Radiosensitivity (LDHRS) was investigated. In agreement with previous reports, the LDHRS was abolished by an acute priming dose of 0.3 Gy given 6 h prior to the challenge dose. When the interval between the priming- and challenge-doses was increased to 24 h, LDHRS was restored. However, when the priming dose was delivered at a low dose-rate (0.3 Gy/h), the abolition of LDHRS that again appeared 6 h following the priming dose, now persisted for intervals of up to 14 weeks between doses even after 28 passages during the 14 weeks.

The Elimination of Low-Dose Hyper-radiosensitivity by Transfer of Irradiated-Cell Conditioned Medium Depends on Dose Rate

Abstract Edin, N. J., Sandvik, J. A., Olsen, D. R. and Pettersen, E. O. The Elimination of Low-Dose Hyper-radiosensitivity by Transfer of Irradiated-Cell Conditioned Medium Depends on Dose Rate. Radiat. Res. 171, 22–32 (2009). Irradiation of T-47D cells with 0.3 Gy delivered by a 60Co source at a low dose rate of 0.3 Gy/h abolished low-dose hyper-radiosensitivity (HRS) for at least 14 months (with continuous cell culturing), while the same dose administered acutely (40 Gy/h) eliminated HRS for less than 24 h. Medium transferred from the low-dose-rate primed cells (low-dose-rate ICCM) to unirradiated cells eliminated HRS in recipient cells even if the donor cells had been cultivated for 14 months after the priming dose. Thus low-dose-rate priming activates mechanisms that involve modification or induction of a factor in the medium. This factor affects unirradiated cells in such a way that HRS is eliminated in cells exposed to medium from the primed cells. However, only cells directly exposed to low-dose-rate radiation induce or modify the putative factor, since unirradiated cells that were exposed to low-dose-rate ICCM regained HRS within 2 weeks of cultivation in fresh medium. The ability of ICCM to eliminate HRS in recipient cells is dependent on dose rate. However, an increase in clonogenic survival was observed in cells receiving only medium transfer without subsequent irradiation that was independent of dose rate.

Mechanisms of the elimination of low dose hyper-radiosensitivity in T-47D cells by low dose-rate priming.

Purpose: To investigate the mechanisms of elimination of low-dose hyper-radiosensitivity (HRS) in T-47D cells induced by 0.3 Gy low dose-rate (LDR) priming. Materials and methods: The mitotic ratio was measured using mitotic marker histone H3 phosphorylation in LDR primed as well as untreated T-47D cells. The HRS response in unprimed cells receiving medium which was irradiated after being harvested from unprimed cells was measured with or without serum present during cell conditioning. 4,6-benzylidene-D-glucose (BG) was used to inhibit protein synthesis during LDR priming. Results: LDR primed T-47D cells were HRS-deficient and showed a decrease in mitotic ratio with increasing dose while unprimed, i.e., HRS-competent T-47D cells, showed no decrease in mitotic ratio for doses in the HRS-range. HRS was eliminated in LDR primed cells, in cells receiving medium transfer from LDR primed cells, and in cells receiving LDR irradiated medium harvested from unprimed cells. The efficacy of the transferred medium depended on the presence of serum during cell conditioning. LDR priming eliminated HRS even in the presence of protein synthesis inhibitor BG. Conclusions: LDR priming of T-47D cells as well as LDR priming of medium conditioned on T-47D cells induce a factor in the medium which cause the early G2-checkpoint to be activated in recipient cells by doses normally in the HRS dose-range.

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.

Separation of two sub-groups with different DNA content after treatment of T-47D breast cancer cells with low dose-rate irradiation and intermittent hypoxia

Background Previous studies have shown that combined treatment with internal ultra-low dose-rate irradiation selectively inactivated hypoxic T–47D breast cancer cells after three to five weeks of treatment. However, 2–3% of the hypoxic cells were found to survive and restart proliferation upon re-oxygenation. Purpose To investigate the metastatic potential and characteristics of radiosensitivity of these surviving cells, named T – 47DS. Material and Methods The T – 47DS cells were grown in ambient air without irradiation. A cloning experiment identified two sub-groups with different DNA content ( T - 47 D S C 1 and T - 47 D S C 2 ). Furthermore, radiosensitivity and presence of hyper-radiosensitivity (HRS) was measured by Co-60 challenge irradiation and relative migration was determined by scratch assays. Results The two subpopulations of T – 47DS had different DNA content; one had abnormally high DNA content ( T - 47 D S C 1 ) and one had DNA content similar to wild-type T–47D cells ( T - 47 D S C 2 ). HRS was surprisingly present in cells of the cloned population T - 47 D S C 1 , but was absent in cells of both T - 47 D S C 2 and T – 47DS. The radio response of T – 47DS, T - 47 D S C 1   and   T - 47 D S C 2 at higher radiation doses were similar to that of T-47D cells, and neither subpopulation showed increased migration compared with wild-type T–47D. Conclusion No increase in the risk of metastasis was found and only slight changes in radiosensitivity in response to conventional clinical doses was observed. Thus, the data suggest that if ultra-low dose-rate irradiation is used for targeting the hypoxic tumor fraction, conventional high dose-rate irradiation can be used to eradicate eventual surviving cells as well as cells in the well oxygenated areas of the tumor.

Monte Carlo simulations of a low energy proton beamline for radiobiological experiments

Abstract Background: In order to determine the relative biological effectiveness (RBE) of protons with high accuracy, radiobiological experiments with detailed knowledge of the linear energy transfer (LET) are needed. Cell survival data from high LET protons are sparse and experiments with low energy protons to achieve high LET values are therefore required. The aim of this study was to quantify LET distributions from a low energy proton beam by using Monte Carlo (MC) simulations, and to further compare to a proton beam representing a typical minimum energy available at clinical facilities. Materials and methods: A Markus ionization chamber and Gafchromic films were employed in dose measurements in the proton beam at Oslo Cyclotron Laboratory. Dose profiles were also calculated using the FLUKA MC code, with the MC beam parameters optimized based on comparisons with the measurements. LET spectra and dose-averaged LET (LETd) were then estimated in FLUKA, and compared with LET calculated from an 80 MeV proton beam. Results: The initial proton energy was determined to be 15.5 MeV, with a Gaussian energy distribution of 0.2% full width at half maximum (FWHM) and a Gaussian lateral spread of 2 mm FWHM. The LETd increased with depth, from approximately 5 keV/μm in the entrance to approximately 40 keV/μm in the distal dose fall-off. The LETd values were considerably higher and the LET spectra were much narrower than the corresponding spectra from the 80 MeV beam. Conclusions: MC simulations accurately modeled the dose distribution from the proton beam and could be used to estimate the LET at any position in the setup. The setup can be used to study the RBE for protons at high LETd, which is not achievable in clinical proton therapy facilities.

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

Abstract 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).