In vitro engineering of human 3D chondrosarcoma: a preclinical model relevant for investigations of radiation quality impact
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The benefit of better ballistic and higher efficiency of carbon ions for cancer treatment (hadron-therapy) is asserted since decades, especially for unresectable or resistant tumors like sarcomas. However, hadron-therapy with carbon ions stays underused and raises some concerns about potential side effects for patients. Chondrosarcoma is a cartilaginous tumor, chemo- and radiation-resistant, that lacks reference models for basic and pre-clinical studies in radiation-biology. Most studies about cellular effects of ionizing radiation, including hadrons, were performed under growth conditions dramatically different from human homeostasis. Tridimensional in vitro models are a fair alternative to animal models to approach tissue and tumors microenvironment.
By using a collagen matrix, standardized culture conditions, physiological oxygen tension and a well defined chondrosarcoma cell line, we developed a pertinent in vitro 3D model for hadron-biology studies. Low- and high-Linear Energy Transfer (LET) ionizing radiations from GANIL facilities of ~1 keV/μm and 103 ± 4 keV/μm were used respectively, at 2 Gy single dose. The impact of radiation quality on chondrosarcoma cells cultivated in 3D was analyzed on cell death, cell proliferation and DNA repair.
A fair distribution of chondrosarcoma cells was observed in the whole 3D scaffold. Moreover, LET distribution in depth, for ions, was calculated and found acceptable for radiation-biology studies using this kind of scaffold. No difference in cell toxicity was observed between low- and high-LET radiations but a higher rate of proliferation was displayed following high-LET irradiation. Furthermore, 3D models presented a higher and longer induction of H2AX phosphorylation after 2 Gy of high-LET compared to low-LET radiations.
The presented results show the feasibility and usefulness of our 3D chondrosarcoma model in the study of the impact of radiation quality on cell fate. The observed changes in our tissue-like model after ionizing radiation exposure may explain some discrepancies between radiation-biology studies and clinical data.
KeywordsSW1353 Cell Articular Chondrocytes Linear Energy Transfer Relative Biological Effectiveness Collagen Scaffold
3D cartilage model
3D chondrosarcoma model
Bone morphogenic protein-2
Bovine serum albumin
Colony forming efficiency
Lethal dose for 10 % survival
Lethal dose for 37 % survival
Dulbecco's Modified Eagle's Medium
DNA Double strand break
Ethylene Diamine Tetraacetic Acid
Evaluation of Relative Biological Effectiveness
Fetal Bovine Serum
Glyceraldehyde 3-Phosphate dehydrogenase
Hematoxylin, erythrosine, safran
Linear energy transfer
Mesenchymal stem cells
Relative biological effectiveness
Sodium Dodecyl Sulfate-Poly-Acrylamide Gel Electrophoresis
Standard error on the mean
Surviving fraction after a 2 Gy irradiation
Spread-out bragg peak
Tissue protein extraction reagent
Emerging protocols of radiation-therapy (RT) with charged particles (protons or heavier ions than helium ions), in advanced medical facilities have widely changed the way of thinking about local tumor control and impact on healthy tissues. Indeed, charged particle-therapy (hadron-therapy) has the advantage of an excellent beam ballistic and a minimal exit dose after energy deposition in the target volume, and hence better sparing of critical structures in the vicinity of the tumor . Unlike photons, protons and heavy ions exhibit a depth-dose distribution profile characterized by the Bragg peak, a sharp rise in energy deposition at the end of their range with a steep dose falloff downstream. However, the ratio of dose at the Bragg peak to that in the entrance region is higher for heavy ions . Furthermore, compared to photons and protons, heavy ions have a higher Linear Energy Transfer (LET). Because high-LET radiation is densely ionizing, the correlated DNA damages within one cell occur more often so that it becomes more difficult for the cell to repair the damage, leading to a markedly increased efficiency of cell killing. In addition, heavy ions have less dependency on cell cycle and oxygen tension. Indeed, a particle beam with a high-LET (LET ~100 +/− 20 keV/μm) is required to meet an optimal biological effectiveness . Thus, RT with heavy ions such as carbon ions represents an attractive radiation modality, which combines the physical advantages of protons, with a higher radiobiological effectiveness. Thanks to such improved biological effectiveness, these technologies are expected to reduce frequency and severity of radiation morbidity. However, the tremendous amount of combination of radiation quality (LET, energy, dose rate, dose) and tissue biological status (co-morbidity factors, genetic background, O2 tension) does not simplify the building of a relevant model for exposure of healthy tissues or tumors during RT . Therefore, it is necessary to develop new tools in order to optimize the use of hadron beams in cancer therapy either in the development of new instruments for beam control and dosimetry or in the understanding of the biological effects of hadrons on healthy tissue and various kinds of tumor.
Chondrosarcoma (CHS) is a malignant skeletal tumor with cartilaginous differentiation (dissimilar from other primary skeletal tumors) and represents the second most common primary bone tumor in adults, generally arising in the fourth decade. It is a heterogeneous group of tumors that have in common the production of chondroid matrix. Conventional CHS subgroup represents ~ 85 % of total cases and can be subdivided in low-grade (I), intermediate-grade (II) or high-grade (III) based on histology . Primary treatment is surgical but, due to the location of tumors close to critical structures (abdomen, cranial and spinal nerves), the complete resection is rarely possible. Indeed, CHS is considered as a chemo- and radiation-resistant cancer, needing high dose RT in inoperable or incompletely resected tumors . Hadron-therapy has been applied to the treatment of low- and intermediate-grade CHS at different locations, with very promising results [2, 5, 6]. However, prospective randomized trials comparing different RT modalities are still needed to validate the superiority of one treatment for a given indication. The understanding of the impact of low- versus high-LET beams on normal and tumor tissues is, then, important to enhance the knowledge and serve clinical use of hadrons.
To date, most current CHS animal models consist in the subcutaneous xenografting of CHS cell lines or human tumor tissue . Three orthotopic CHS mouse models were published recently [8, 9, 10] using CHS human cell lines, but there is no transgenic mouse model for this disease. Considering the lack of radiation-biology studies on CHS, in contrast with the existence of a subset of medical data confirming the effectiveness of hadrons, and the absence of an in vivo reference model, we developed a new CHS three-dimensional (3D) model in order to mimic in vivo microenvironment. Indeed, over the past sixty years, two-dimensional monolayer cells (2D) cultured in atmospheric oxygen tension (20 % O2, referred as normoxia) have been considered as a gold standard in cell biology and more specifically in radiation-biology studies. However, the 3D environment  and oxygen tension  have a major impact on cellular response to ionizing radiations (IRs). Basic 3D cultures of CHS cells have been previously used for drug testing but chondrogenic properties of type I/III collagen , and physioxic culture conditions were not taken into account as demonstrated previously [14, 15]. Indeed, in a previous study, P. Galera’s team proposed a new 3D cartilage model (3DCaM), relevant for arthritis analysis . They used a 3D scaffold composed of cross-linked type -I and -III collagen, and successfully handled this matrix with Articular Chondrocytes (AC), isolated from human donors. As compared with conventional in vitro 2D culture, they showed the advantages of this 3D scaffold, in association with chondrogenic factors and physiological oxygen tension (2 % O2, referred as physioxia), to allow cell re-differentiation and natural cartilage matrix synthesis. It should be noted that cartilage is the only avascular tissue of human body explaining the low O2 tension of this tissue [16, 17].
Using the most characterized grade II CHS cell line (SW1353), a standardized chondrocyte medium, a chondrogenic factor (BMP-2), physiological oxygen tension (2 % O2) and the same collagen scaffold [15, 18], we report here the first 3D CHS model (3DCM) applied to radiation-biology studies. We used two different IRs; accelerated 18O ion beam as a high-LET radiation, comparable to the LET of carbon ions beam delivered into the tumor volume (Spread-Out Bragg Peak, SOBP) during hadron-therapy , and X-rays as a low-LET radiation (control). We used a single 2 Gy dose and LET distribution profile of ions was calculated in order to ascertain a homogenous irradiation of 3DCM. Radiation-induced cell death was assessed with our 3DCM and canonical clonogenic assay in 2D culture as a reference. Ki67 proliferation index and gamma-H2AX kinetic were carried out to demonstrate the feasibility and the proof of usefulness of 3DCM in hadron-biology and the impact of radiation quality on proliferation and DNA double strand breaks (DSBs) repair.
Reagents and antibodies
SW1353 CHS cell line, human Articular Chondrocytes (AC) and the following culture media were purchased from CellSystems (Troisdorf, Germany); Chondrocyte Growth Medium w/o Phenol Red (#411PR-500), Chondrocyte Basal Medium w/o Phenol Red w/o FBS (#410PR-500) and Chondrocyte Growth Medium w/o Phenol Red w/o FBS (#411FPR-500). Collagen scaffolds were bought from Symatese Biomateriaux (Chaponost, France), BMP-2 (Bone Morphogenic Protein-2) from R&D Systems (Minneapolis, USA), Toxilight™ assay from Lonza (Basel, Switzerland), Digitonin from Promega (Madison, USA), T-PER (Tissue Protein Extraction Reagent), Halt Protease Inhibitors Cocktail 100 X, Halt Phosphatase Inhibitors single-use Cocktail 100 X, anti-GAPDH (#11335232), anti-rabbit HRP-coupled (#31460) and anti-mouse HRP-coupled (#31430) antibodies from Thermoscientific (Waltham, USA), anti-H2AX phosphoserine 139 (#05-636) and anti-Ki67 (#AB9260) antibodies, ECL classico/crescendo and Accutase™ from Merck Millipore (Darmstadt, Germany), DAB (Diaminobenzidine) from Life technologies (Carlsbad, USA).
All the data reported in this manuscript have been collected from commercially available human healthy chondrocytes and CHS cells used in compliance with the Helsinki Declaration. CellSystems Company (Troisdorf, Germany) and its supplier, Cell Applications Company (San Diego, USA), follow bioethics guidelines to comply with the Helsinki Declaration.
The use of human cells by our Institute and Laboratory was approved by the French Ministry of Research under the CODECOH reference: DC-2008-228.
From Dulbecco's Modified Eagle's Medium (DMEM) supplemented with 2 mM L-glutamine, 10 % FBS and antibiotics (100 U/mL of penicillin V, 100 μg/mL of streptomycin), cells were gradually adapted to a standardized Chondrocyte Growth Medium (#411PR-500). Cells were checked for Mycoplasma contamination and aliquots frozen for further experiments. During cell expansion (2D culture), cells were seeded at 1.3 × 104 cells/cm2 in 75 cm2 culture flasks and maintained at 37 °C in a humid atmosphere (95 % air, 5 % CO2). Instead of trypsin, Accutase™ was used as a cell detachment reagent. SW1353 were passaged twice a week, not more than ten times.
P. Galera’s team model was adapted for radiation-biology experiments but instead of DMEM, we used a 3D Chondrocyte Medium (#411FPR-500) supplemented with 2 % FBS. Briefly, cells were seeded onto the scaffold at 4 × 105 cells/scaffold in 96-well culture plates and incubated at 37 °C and 5 % CO2. One hour later, 100 μL of the previous medium supplemented with BMP-2 (50 ng/mL) were added to the well and cells were incubated in physioxia (2 % O2) in a Heracell™ 150i Tri-Gas Incubator, for 7 days to obtain a 3DCM. The medium was changed twice a week.
Cell cycle analysis
Cell cycle distribution analysis was performed in 2D culture. SW1353 cells were plated at subconfluency in T25 culture flasks then placed in the incubator for 6 h. Cells were then harvested and centrifuged at 800 rpm for 5 min. Cell pellet was washed in PBS, fixed in ethanol 75° then stored at 4 °C until analysis. Briefly, cells were centrifuged at 2000 rpm for 5 min, and the cell pellet resuspended in PBS before staining. The remaining pellet was gently resuspended in 500 μL DNA Prep Stain and 50 μL DNA Prep LPR (DNA Prep Reagent kit, Beckman Coulter). Samples were incubated in the dark for 15 min and a minimum of 1 × 104 cells per sample were analyzed using GALLIOS flow cytometer (Beckman Coulter, Passadena, USA). FlowJo analyzing software (Ashland, USA) was used. Experiments were repeated four times and data expressed as mean ± Standard Error on the Mean (S.E.M.).
Low-LET radiation exposure was performed, either at the CLCC (Centre de Lutte Contre le Cancer) François Baclesse (Caen, France) or at Cyceron facility (Caen, France). We used respectively, a Saturne 15 (15 MV, 6 mA, Siemens) medical linear accelerator producing X-rays or an X-RAD 225 Cx (225 kV, 13 mA, PXi) research X-rays generator intended to cellular and small animal irradiation. The X-RAD 225 Cx is characterized by a first half value layer of 0.9 mm of copper. X-rays of this beam produce low energy secondary electrons with dE/dX ranging from 0.26 to 2.25 keV/μm with a mean value of 1.65 keV/μm (instead of 0.2 keV/μm for MV beams). Absolute dosimetry of the irradiator was performed following AAPM TG-61 protocol  and the dose delivered to cellular samples was measured thanks to ionization chamber measurements and thermo luminescent dosimeters (TLD). Except for survival curves, the canonical single dose fraction in conventional radiotherapy of 2 Gy was used for all experiments at a 2 Gy/min dose rate.
Irradiation of 3DCM was performed as follows: the 96-well plates containing the 3DCM were sealed with parafilm in order to maintain 2 % O2 tension during irradiation and put on the horizontal plate within the X-ray generator (Fig. 2a). Mock-irradiated cells were handled in the same conditions without being irradiated. Following irradiation, medium was changed. Samples were collected at different time points, rinsed once in Dulbeccos’s PBS then stored either at −80 °C for western blot analysis or formalin-fixed for immunohistochemistry-paraffin (IHC-p). Culture supernatant (100 μL) was also collected and stored at −80 °C for cell toxicity assay.
High-LET radiation exposure was performed using the D1 IRABAT high-energy scanning beam line at the Grand Accélérateur National d’Ions Lourds (GANIL, Caen, France). The dosimetry and calibration beam was done by CIMAP (Centre de Recherche sur les Ions, les Matériaux et la Photonique) as previously described [22, 23]. Accelerated 18O ions (50 MeV/a, < 0.1 nA) were used at a dose rate of ~1 Gy/min which corresponded to a mean fluency of 1.22 × 105 ions/ (cm2.s). A minimum of 30 s irradiation time was used in order to ensure a homogeneous dose on each sample. Except for survival curves, 2 Gy dose was used for all the samples.
Irradiation of 2D cells was performed as follows: cells were prepared similarly to X-ray irradiation protocol (see above). Subconfluent adherent cells were irradiated in an upright position at Room Temperature (Fig. 2b). Control flasks were mock-irradiated. Following irradiation, cells were maintained in the Tri-Gas Incubator (2 % O2) in fresh Growth Medium previously balanced with 2 % O2 for further cell survival experiments.
3DCM were maintained in a vertical position in a 2 mL polypropylene tube (Eppendorf®), with a sterile sample holder consisting of a glass cylinder as shown in Fig. 2b. The tube was filled with 3D Chondrocyte Medium previously balanced with 2 % O2 and irradiated in an upright position. In this configuration, collagen scaffolds are laid-out against the polypropylene tube thanks to the sample holder. Following irradiation, medium was changed and samples collected similarly to low-LET irradiated samples.
LET distribution profile in the collagen scaffold
CFE: Colony Forming Efficiency (2D culture)
Clonogenic assessment in 2D was done by colony forming efficiency assay as previously described [28, 29]. Subconfluent 2D cells were irradiated as described above. The cells were left untreated from 12 to 16 h post-irradiation, then trypsinized, counted and plated in six-well plates (BD Falcon™) at two densities (100 or 1000 cells). The cells were grown for 12 days without medium change, then fixed and stained with crystal violet (3 % w/v in 20 % ethanol solution). Only colonies of 50 cells or more were scored. The experimental survival curve data were fitted to the linear quadratic equation : − Ln (S) = αD + βD2. S is the surviving fraction at a given dose D. This calculation takes into account the mock-irradiated condition. α and β are constants determined by fitting the data to the model using the nonlinear regression program of the Prism software package (Graphpad Software, San Diego, CA). D10 (lethal dose for 10 % survival), D37 (lethal dose for 37 % survival) and SF2 (surviving fraction after a 2 Gy irradiation) values were determined from the fitted curve. Experiments were performed in triplicate and repeated from 2 to 6 times according to beam time availability. α and β are expressed as mean ± S.E.M.
Canonical Relative Biological Effectiveness (RBE) was calculated by dividing D10 of low-LET radiation by the corresponding parameter following high-LET irradiation. RBE was also calculated using D37.
Cell toxicity assay
Cell toxicity was assessed on 3DCM culture medium using Toxilight™, a bioluminescent cytotoxicity assay designed to quantitatively measure the release of Adenylate Kinase (AK) from damaged cells. Manufacturer’s instructions were followed. Briefly, in a 96-white well plate (Greiner bio-one®), 20 μL of thawed cell culture supernatants were mixed to 100 μL/well of freshly prepared AK working solution. Plates were incubated for 5 min at room temperature before measurement. Flexstation 3 (Molecular Devices, Sunnyvale, USA) at the Proteogen plateform of Université Caen Basse-Normandie (UCBN) was used and programmed to 1 s integrated reading of appropriate wells. Digitonin detergent was used as a positive control (65 pM) as recommended by the manufacturer. Data were collected in RLU (Relative Light Unit).
Immunohistochemistry-paraffin (IHC-p) staining
Formalin fixed 3DCM underwent a classical immunohistochemistry protocol (for paraffin sections), but manually performed to maintain scaffold integrity and avoid material loss. 3DCM were dehydrated in a graded series of ethanol on the first day and paraffin-soaked overnight. They were then paraffin-embedded in an upright position at the Pathology department of the CLCC François Baclesse. Eight μm sections were cut with a microtome and mounted on Superfrost™ Plus microscope slides, precisely, to avoid counting the same cells. The slides were dried overnight at 37 °C and stored at room temperature.
IHC staining was carried out overnight in a humidified chamber using monoclonal antibody directed against Ki67 diluted to 1/100 in 1 % BSA (Bovine Serum Albumin) in 0.5 % Tween PBS (TPBS). Then, 1 h incubation with HRP-conjugated anti-rabbit secondary antibody (diluted to 1/100 in 1 % BSA in TPBS) was done. Sections were then revealed with DAB and mounted using Eukitt mounting medium. No antigen retrieval was performed and as control, a slide without primary antibody incubation was realized.
The slides were observed with a Vanox-S Olympus light microscope (Tokyo, Japan) using a 40x lens. For each time point, around 200 nuclei were counted by the same experimenter, choosing 2 to 3 fields per slide (on a total of 5–10 slides). Only clearly stained cells were scored as DAB positives. Mock-irradiated sample is expressed as mean ± S.E.M from two independent experiments.
HES (Hematoxylin, Erythrosine, Safran) classical staining was also used to assess general organization of the collagen scaffold and cell distribution. Image acquisition was achieved, either with a Nikon Coolscope scanner (Tokyo, Japan) at the Pathology department of CLCC François Baclesse or Aperio Scancope CS scanner (Leica biosystems, Nussloch, Germany) at HIQ plateform of UCBN (Caen, France). Representative images were shown in the figures.
Cell lysis protocol and western blotting
3DCM were disrupted using the following cell lysis protocol at 4 °C. Glass beads of 100 μm diameter (25 mg, Dominique Dutcher, Brumath, France) were added to a round-bottomed 2 mL Eppendorf tube and mixed with 40 μL of a freshly prepared lysis buffer [T-PER (1 mL), NaCl (850 mM), Halt Protease Inhibitors Cocktail (2X v/v), EDTA (2X v/v), Halt Phosphatase Inhibitors single-use Cocktail (2X v/v)]. One sample was then quickly thawed and mixed with lysis buffer and glass beads at 4 °C for 15 min using a disruptor GENIE™ (Dominique Dutcher). This cell lysis step was followed by addition of 10 μL Laemmli buffer (5X) and 5 min mix in the disruptor, to extract the proteins of the sample. This was followed by a protein denaturation step (sample heated twice at 100 °C for 5 min). After a quick centrifugation, 40 μL of the supernatant were collected. The sample underwent a second cycle of protein extraction-denaturation and 10 μL of extract were collected again. To estimate an extraction yield, the above protocol was performed twice on the same test sample, and the ratio of both extracts was calculated using ECL signal (the yield is expressed as mean ± S.E.M from two experiments).
Half of the extracted sample (25 μL) underwent SDS-Poly-Acrylamide 10 % Gel Electrophoresis (SDS-PAGE) and was transferred to a nitrocellulose membrane. Membranes were blocked for 1 h in 5 % milk in 0.05 % tween-TBS (TTBS) and incubated overnight at 4 °C with anti-H2AX phospho-serine 139 (gamma-H2AX) or anti-GAPDH as a loading control, both diluted at 1/1000 in 1 % milk TTBS. Membranes were then incubated for 1 h at room temperature with anti-mouse secondary peroxidase-conjugated antibodies (1/5000 diluted in 1 % milk TTBS). Detections were assessed on X-ray films (GE healthcare) using the ECL method. Image J software was used to quantify the non-saturated signals. Data were expressed as relative amount of gamma-H2AX compared to GAPDH. Evaluation of RBE (ERBE) was expressed as the relative amount of gamma-H2AX protein following high-LET irradiation divided by the same parameter post low-LET irradiation at the same time point.
Cell lysis and protein extraction using 2D cells were performed as described for 3DCM. Cell counting was used to adjust lysis and Laemmli buffer volumes. Around 0.25 million cells per sample were used to perform the western blot.
Results and discussion
SW1353 cells characterization
The therapeutic use of hadrons has mainly focused on low- and intermediate-grade CHS . In this study, we focused on intermediate-grade (II) CHS as they show relative radioresistance to photons, a metastatic potential and high recurrence rate but still maintain a cartilage phenotype . SW1353, JJ012 and CH3573 are currently the most characterized conventional grade-II CHS cell lines . Among them, SW1353 is the most extensively used and is considered as the gold standard among other cells. Indeed, 142 articles were found in Pubmed library: [SW1353 OR HTB-94] compared to 39 articles for [JJ012] or 1 article for [CH3573]).
SW1353 cells were adapted from DMEM to a full standardized medium then amplified in a standard cell incubator (20 % O2 tension, normoxia). However, 3DCM were cultivated under 2 % oxygen tension in order to mimic human in situ microenvironment for cartilage [16, 17]. Such condition, referred here as physioxia, was then applied to 2D irradiations or experimental assays. Untreated SW1353 cells in 2D culture show a doubling time of 23 h and a usual cell cycle distribution with 42.2 % (±0.9), 35.2 % (±1.2) and 22.5 % (±0.8) of cells collected in G0/G1, S-phase and G2/M phase’s, respectively (Additional file 1: Figure S1).
3DCM were prepared by seeding 4 × 105 cells in each scaffold and culturing them 7 days in physioxia as described before . After 7 days of maturation, about 1.5 × 105 of the seeded cells did not attach into the scaffold and were discarded by PBS washing. Cell distribution of attached cells throughout the scaffold was then analyzed. A representative image of a paraffin-embedded, transversally cut, HES colored slide of a 3DCM is shown in Fig. 1. The three magnified images (bottom) showed a homogenous cell distribution in the proximal (bottom, left), internal (bottom, middle) and distal (bottom, right) zones of the scaffold (Fig. 1). This was comparable with the cell distribution observed in 3DCaM seeded with AC using Scanning Electron Microscopy  or IHC and light microscopy . Grade II CHS as SW1353 cell line show poor cellularity in histological sections . Thus, under cell culture conditions described here, we were able to grow a homogeneous 3DCM miming an intermediate grade CHS tissue cellularity.
Evaluation of the intra-individual counting variability of the Ki67 proliferation index in the 3DCM
Ki67 index (%)
Standard deviation (%)
Ki67 proliferation index (%) 96 and 168 h following low-LET or high-LET irradiation
Time post-irradiation (hrs)
33 ± 4
Ki67 proliferation index (%) in the 3DCaM using two healthy male donnors
Ki67 proliferation index in the 3DCaM
Donnor 1 (38 years old)
Donnor 2 (51 years old)
Mean ± S.E.M
17.5 ± 4.5
Irradiation set up, dosimetry and LET distribution
X-rays clonogenic assay experiments were performed with a 225 kV irradiator and a 15 MeV accelerator. Survival curve characteristics were almost identical with both devices (standard deviation < 10 %), a result which was previously described with different cell lines . Subsequently, collected data were pooled. As X-rays beams were vertical, 2D culture flasks and 3DCM culture plates were maintained horizontal (Fig. 2a). On the contrary, heavy ion scanning beam from IRABAT line (GANIL) is horizontal (Fig. 2b). Thus, 2D culture flasks and tubes bearing 3DCM were then maintained in an upright position (Fig. 2b). However, in both conditions, flasks and 3DCM were fully filled with medium and irradiated in physioxia. Except for survival curves, SW1353 cells as 2D cultures or 3DCM were irradiated with 2 Gy of either X-rays (low-LET) or 18O ions (high-LET) radiation. These two conditions were chosen to mimic a canonical fraction of conventional radiotherapy (low-LET) versus a fraction of hadron-therapy with carbon ions (high-LET). Indeed, the LET of 18O accelerated ions (50 MeV/a) used in this study is approximatively of 103 ± 4 keV/μm which is comparable to the LET of SOBP of a carbon ions therapeutic beam .
LET values of 18O ions in the proximal and distal zones and into the 3DCM
Into the 3DCM
85.91 ± 0.38
99.87 ± 0.21
109.82 ± 0.57
96.27 ± 0.32
107.24 ± 0.11
122.97 ± 0.67
Cell survival and proliferation post-irradiation
Radiation survival curve characteristics for SW1353 cells cultured in 2D
15 MeV 225 kV
0.145 ± 0.036
0.037 ± 0.005
103 ± 4
2.567 ± 0.217
Cellular toxicity assay in 3DCM models following irradiation or digitonin treatment
Ratio of luminescence relative to mock-treated sample
Digitonin 65 pM
Post-irradiation gamma-H2AX repair kinetic
Reproducible and effective protein extraction from the 3DCM was a technical challenge, as cellular protein extraction from collagen scaffold was not efficient using a standard protocol. Thus, we developed a new protocol using a cell disruptor system and specific glass beads, as described above. This protocol allows us to calculate an extraction yield of 94 % (±4 %) from the quantification of western blot signal, showing the reproducibility and capacity of our protocol to extract and analyze low abundant proteins.
On the contrary, following high-LET irradiation, gamma-H2AX induction still increases after 1 h post-irradiation to reach a maximal at 12 h with a 10-fold induction compared to the mock-irradiated sample (Fig. 6b). The gamma-H2AX expression then decreases slowly. This higher induction could be explained by the fact that high-LET radiations trigger more complex DNA damages that are harder to repair, compared to low-LET radiation as it has been already shown [42, 43, 44]. However, with 3DCM we show here that such higher gamma-H2AX induction is wider, meaning probably a delayed DNA repair kinetic due to our physioxic cell culture conditions. Furthermore, gamma-H2AX induction was also measured on 2D culture conditions at the same time points post-irradiation (Additional file 4: Figure S4 and Additional file 5: Figure S5). Using GAPDH as loading control, we measured ~ 3-fold gamma-H2AX induction with low-LET radiation, 1 h post-irradiation compared to mock-irradiated sample. Following this induction, low-LET irradiated samples display a two phase decrease with a fast first step until 3 h post-irradiation and then a slower decrease from 6 h to 24 h (Additional file 4: Figure S4). High-LET irradiated samples display a 3-fold gamma-H2AX induction 1 h post-irradiation, similarly to low-LET exposure. However, the decrease is very slow and only partial as a 2-fold induction is still measured 96 h after exposure (Additional file 5: Figure S5). Such delayed gamma-H2AX induction was recently described for high-LET radiation  and is consistent with well-known delayed DSB repair measured after high-LET exposure in 2D cell culture .
Finally, we propose a new evaluation marker of biological effectiveness named Evaluation of RBE (ERBE) by calculating the ratio of the gamma-H2AX induction following high-LET irradiation compared to low-LET as a function of time. Indeed, in the present study ERBE at 12, 24 and 48 h following irradiation show a significant decrease as estimated respectively to 3.0, 1.5 and 1.0 in 3DCM (Fig. 6). This marker may be useful to monitor the impact of radiation quality in 3D models for whom collecting irradiated cells for canonical clonogenicity is impossible. Furthermore, analysis of ERBE in a time course manner after radiation exposure may enlighten some undiscovered aspect of tissue response to hadron-therapy, especially for RT hypo-fractionation clinical protocols.
In the present study we report, for the first time, a 3D physioxic CHS model applied to radiation-biology studies. We showed its usefulness for studying radiation quality impact on CHS in a controlled microenvironment that mimics in vivo tissue homeostasis after therapeutic radiation exposure. LET distribution calculation demonstrated a homogenously ions irradiation through the 3DCM. We describe here differential cellular responses (proliferation, gamma-H2AX induction kinetic) following low- versus high-LET radiation, using the same dose (2 Gy). The discrepancies found between 2D culture and 3DCM response to radiation, especially for high-LET exposure, may be explained by the reconstitution of a tissue-like microenvironment in 3DCM. Indeed, cells embedded in extracellular matrix change their whole metabolism  and may display specific delayed response as found for gamma-H2AX induction [41, 42]. Furthermore, high-LET irradiations of 3DCM show a particular gamma-H2AX induction, higher and longer than low-LET. Such specific pattern should be investigated in the future as 3D models may be used to assess new and more relevant indexes of radiation biological efficiency. Indeed, our 3DCM could be used as a validation tool for medical beams using carbon ions or protons in order to evaluate the RBE-LET dependence and measure the variation of RBE along the depth-dose profile of therapeutic proton or heavy ion beams. This issue was pointed out to be critical, at least in proton-therapy by Britten et al., 2013 .
Furthermore, the 3DCM strategy described here could also be applied to human AC or Mesenchymal Stem Cells (MSCs) for radiation quality impact investigations on normal cartilage or MSCs differentiation after therapeutic radiation exposure as it has been previously studied for low-LET radiations . Such models could also be transplanted in nude mice, as it has been previously performed with AC .
Finally, in this study, we focused on targeted effects of IRs on CHS cultivated in 3D. However, non-targeted effects of IRs have been previously suggested to play a role in radiation-resistance of CHS to low-LET radiations . Further studies with 3DCM using innovative tools will help understand the radiation-resistance of CHS in clinics .
This work was supported by DSV-CEA (AAP Radiobiologie, Direction des Sciences du Vivant, Commissariat à l’Energie Atomique et aux Energies Alternatives, Fontenay aux Roses, France) and Conseil Régional de Basse-Normandie (programme Emergence, Caen, France). The authors are responsible for the study design, the writing of the report and the decision to submit the article for publication. DHH is supported by a PhD scholarship (Irtelis Graduate Program of DSV-CEA). SB is supported by the EU Erasmus program in collaboration with the University of Pavia (Italy) Master’s school.
We thank A. Batalla and P. Chevallier for technical assistance with Saturn medical accelerator (CLCC François Baclesse), A-M. Frelin-Labalme for X-RAD 225 Cx dosimetry and CIMAP physicists E. Balanzat, Y. N’Gnono, F. Durantel and A. Cassimi for IRABAT heavy ions beam calibration and dosimetry (GANIL). We are also grateful to Pr C. Bor, J. Marnay and C.Bageot for assistance with IHC-p (pathology laboratory, CLCC François Baclesse), M. Guillamin for technical assistance with flow cytometry (UCBN) and N. Elie for assistance with imaging (HIQ, UCBN).
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