A STAT3-decoy oligonucleotide induces cell death in a human colorectal carcinoma cell line by blocking nuclear transfer of STAT3 and STAT3-bound NF-κB
- Inès Souissi1, 2,
- Imen Najjar6, 7,
- Laurent Ah-Koon1, 2,
- Pierre Olivier Schischmanoff†1, 2, 3,
- Denis Lesage†1, 2,
- Stéphanie Le Coquil1, 2,
- Claudine Roger4,
- Isabelle Dusanter-Fourt6, 7,
- Nadine Varin-Blank1, 2,
- An Cao8,
- Valeri Metelev5,
- Fanny Baran-Marszak1, 2, 4 and
- Remi Fagard1, 2, 3Email author
© Souissi et al; licensee BioMed Central Ltd. 2011
Received: 25 February 2011
Accepted: 12 April 2011
Published: 12 April 2011
The transcription factor STAT3 (signal transducer and activator of transcription 3) is frequently activated in tumor cells. Activated STAT3 forms homodimers, or heterodimers with other TFs such as NF-κB, which becomes activated. Cytoplasmic STAT3 dimers are activated by tyrosine phosphorylation; they interact with importins via a nuclear localization signal (NLS) one of which is located within the DNA-binding domain formed by the dimer. In the nucleus, STAT3 regulates target gene expression by binding a consensus sequence within the promoter. STAT3-specific decoy oligonucleotides (STAT3-decoy ODN) that contain this consensus sequence inhibit the transcriptional activity of STAT3, leading to cell death; however, their mechanism of action is unclear.
The mechanism of action of a STAT3-decoy ODN was analyzed in the colon carcinoma cell line SW 480. These cells' dependence on activated STAT3 was verified by showing that cell death is induced by STAT3-specific siRNAs or Stattic. STAT3-decoy ODN was shown to bind activated STAT3 within the cytoplasm, and to prevent its translocation to the nucleus, as well as that of STAT3-associated NF-κB, but it did not prevent the nuclear transfer of STAT3 with mutations in its DNA-binding domain. The complex formed by STAT3 and the STAT3-decoy ODN did not associate with importin, while STAT3 alone was found to co-immunoprecipitate with importin. Leptomycin B and vanadate both trap STAT3 in the nucleus. They were found here to oppose the cytoplasmic trapping of STAT3 by the STAT3-decoy ODN. Control decoys consisting of either a mutated STAT3-decoy ODN or a NF-κB-specific decoy ODN had no effect on STAT3 nuclear translocation. Finally, blockage of STAT3 nuclear transfer correlated with the induction of SW 480 cell death.
The inhibition of STAT3 by a STAT3-decoy ODN, leading to cell death, involves the entrapment of activated STAT3 dimers in the cytoplasm. A mechanism is suggested whereby this entrapment is due to STAT3-decoy ODN's inhibition of active STAT3/importin interaction. These observations point to the high potential of STAT3-decoy ODN as a reagent and to STAT3 nucleo-cytoplasmic shuttling in tumor cells as a potential target for effective anti-cancer compounds.
STAT3 belongs to the signal transducers and activators of transcription (STATs) family of transcription factors (TFs) . STAT3 is activated in response to several cytokines and growth factors, including IL-6, epidermal growth factor (EGF), and interferon (IFN) α; STAT3 is also weakly activated in response to other cytokines, including IFNγ. Activation of STAT3 results from the phosphorylation of tyrosine 705, mediated by Janus Kinases (JAK), which are associated to cytokine receptors, and also by the Src and Abelson (Abl) families of protein tyrosine kinases . STAT3 is also phosphorylated on serine 727, sometimes resulting in its activation. Following phosphorylation, STAT3 dimerizes and enters the nucleus by interacting with nuclear import proteins  of the karyopherin/importin family . The importins interact with nuclear localization signals (NLS), one of which is located within the DNA binding domain (DBD) of STAT3 and is thought to be the most efficient [3, 5]. Once in the nucleus, STAT3 activates the transcription of its target genes, including cyclin D1, survivin, VEGF, c-myc, Bcl-xL, and Bcl2 (see  for review). Once released from its DNA targets, STAT3 is dephosphorylated in the nucleus  and exported to the cytoplasm by a CRM1-dependent process . STAT3 has been described as a key regulator of cell survival and proliferation ; its constitutive activation has been observed in many human tumors, including colon, breast, lung, pancreas and prostate cancers, melanoma, head and neck squamous carcinoma, multiple myeloma, mantle cell lymphoma, and glioma [10, 11]. In addition, substituting amino acids located at the STAT3 dimer interface for cysteines yielded a stabilized STAT3 dimer that was able to induce a pseudotransformed phenotype . Thus, its constitutive activation in tumor cells points to STAT3 as a valuable target for attacking tumor cells. Furthermore, despite its essential role in development , STAT3 is not essential for the functioning of mature cells . Some STAT3 inhibitors are not specific, such as curcumin . In contrast, Stattic, which prevents STAT3 dimerization by specifically interacting with its SH2 domain , is highly specific, and efficiently induces tumor cell death [16, 17]. Despite its frequent involvement in cancer, which makes it a highly valuable target for inducing tumor cell death, STAT3 still lacks more specific inhibitors. Besides the SH2 domain, another potential target for highly selective STAT3 inhibitors is its DBD, since it selectively recognizes and binds DNA motifs in target genes. Decoy oligonucleotides (decoy ODNs) containing the TFs' DNA binding consensus sequences selectively inhibit them by binding to the DBD . They can induce, in vitro, the death of tumor cells whose growth depends on these TFs . This has notably been shown for several TFs, including NF-κB [20, 21] and STAT3 [17, 22–24]. STAT3-decoy ODN efficiently induced cell death in mouse xenografts of a head and neck squamous cell carcinoma . One limitation of STAT3-decoy ODN is that despite the different functions of STAT1 and STAT3 in the cell, they recognize very similar DNA targets , with the result that STAT3-decoy ODN can inhibit either one or the other. For example, in the colon carcinoma cell line SW 480, the constitutive activation of STAT3 contributes to cell survival; its inhibition by STAT3-decoy ODN induces cell death. However, the ODN also blocks IFNγ-mediated cell death through STAT1 activation in the same cell line . The actual mechanism through which decoy ODNs inhibit TFs is still unclear. Of the many studies demonstrating decoy ODN-mediated inhibition of TFs such as E2F, NF-κB , CRE and AP1 , none have specifically investigated the subcellular localization required for decoy ODNs to exercise their inhibitory action. A study on AP1 suggested that nuclear entry is required for decoy ODNs to inhibit targeted TFs . Another study showed that a decoy ODN engineered to contain a nuclear localization signal (NLS) could enter the nucleus and efficiently inhibit p53 . It is not clear yet whether these requirements depend on cellular systems or on the TFs that are targeted, since other studies have found that decoy ODNs did not have to enter the nucleus to exert their inhibitory effect [17, 21]. In order to assess their possible use in human cancer, it will be important to understand the mechanism through which the decoy ODNs interfere with TFs and to determine whether nucleo-cytoplasmic shuttling is impaired. In the case of STAT3, constitutive shuttling from cytoplasm to nucleus has been demonstrated [8, 31]. Furthermore, STAT3's localization seems to be predominantly nuclear , indicating that the shuttling mechanism could be a promising target for achieving effective STAT3 inhibition, as previously suggested . Decoy ODNs' mechanism of action on STAT3 was therefore studied to determine whether nucleo-cytoplasmic shuttling was impaired, leading to STAT3 inhibition. Finally, since STAT3 has been reported to interact and synergize with NF-κB  in tumor cells , this study also addresses the functional interplay of NF-κB and decoy ODN.
Cell culture and reagents
SW 480 (colon adenocarcinoma) and MCF-7 (breast cancer) cell lines were grown in DMEM (GibcoBRL, Life technologies, Cergy-Pontoise, France), supplemented with 10% FCS (Lonza, Levallois-Perret, France) 100 U/mL penicillin, 10 μg/mL streptomycin (GibcoBRL), 1 mM sodium pyruvate (GibcoBRL), MEM vitamins 100 × (GibcoBRL) and 5 μg/mL plasmocin (Cayla InvivoGen, Toulouse, France). The KG-1 cells were grown in 10% FCS supplemented IMDM medium (GibcoBRL). For the STAT3 overexpression experiments the plasmid PLZst3α was used. The STAT3 DNA binding domain (DBD)-mutant containing two mutations in the DBD that completely prevented DNA binding but allowed dimerization and nuclear entry, was a kind gift from Dr. C. Horvath (Northwestern University, Chicago, USA) . For some experiments, cells were treated with TNFα (20 ng/ml) (Sigma-Aldrich, Montigny le Bretonneux, France). To enhance STAT3 activation, cells were treated for 1 hr with IL-6 (50 ng/ml) (Sigma). Sodium orthovanadate (100 μM) (stock solution: 100 mM) was from Fischer (Illkirch, France), leptomycin B (LMB) (10 ng/ml) was from Sigma-Aldrich.
For cell infection with lentiviral shRNA, a set of two STAT1-targeting shRNAs that has previously been found to reduce the expression of STAT1  was used and transduced as previously described . Efficiency of infection was verified by measuring GFP by flow cytometry, and the efficacy of the inhibition of the shRNA's inhibition of STAT1 expression was verified by western blotting using a STAT1-specific antibody (Cell Signaling, Ozyme, St Quentin Fallavier, France).
For siRNA STAT3 silencing, the following double stranded siRNA oligonucleotide, previously shown to suppress STAT3 expression in a colorectal cell line , was purchased from Sigma-Aldrich: 5'-AACAUCUGCCUAGAUCGGCUAdTdT-3'; 3'-dTdTGUAGACGGAUCUAGCCGAU-5', along with a universal control set of siRNA (Sigma Aldrich). Cells (105 cells/well; density: 60%) were transfected using polyethylene imine (PEI) with 10 nM siRNA in culture medium without antibiotics. After 48 h or 72 h, cells were harvested and analyzed for annexin V binding by flow cytometry. In control cells, the irrelevant control siRNA was used. All experiments were performed in triplicate.
Preparation of subcellular fractions
Cells (20 × 106) were resuspended in cell lysis buffer containing 20 mM Hepes pH 7.4, 1 mM MgCl2, 10 mM KCl, 0.3% NP40, 0.5 mM DTT, 0.1 mM EDTA and protease inhibitors (CompeteTM, Boerhinger, France), and placed at 4°C for 5 min. The lysates were centrifuged at 14000 g for 5 min at 4°C, and the supernatant containing the cytoplasmic fraction was stored in aliquots at -80°C. The pellets were resuspended in cell lysis buffer adjusted to 20% glycerol and 0.35 M NaCl and placed at 4°C for 30 min. After centrifugation at 14000 g for 5 min at 4°C, the supernatant, containing the nuclear proteins, was stored at -80°C. Protein amounts were determined before use with the micro-BCA protein determination kit (Pierce, Perbio, Brebières, France).
The STAT3-decoy ODNs used were: RHN(CH2)6- CATTTCCCGTAAATCGAAGATTTACGGGAAATG -(CH2)3NHR (hp STAT3-decoy ODN), derived from the serum-inducible element of the human c-fos promoter , and RHN(CH2)6- CATTTGCCACAATCGAAGATTGTGGCAAATG -(CH2)3NHR (hairpin STAT3-decoy mutated ODN) (Sigma-Proligo) where R was either H, FITC or biotin. The decoy NF-κB-ODN consisted of: RNH(CH2)6-CTGGAAAGTCCCTCGAAGAGGGACTTTCCAG-(CH2)3NHR (hairpin decoy NF-κB-ODN) and RHN(CH2)6-TGCAGTCACTACGCGAAGCGTAGTGACTGCA-(CH2)3NHR (hairpin scrambled decoy NF-κB-ODN) where R is either H or biotin. The synthesis of decoy oligonucleotides with R = H has been published elsewhere . For biotin addition, 7-10 nanomoles of the oligodeoxynucleotide bearing 3'- and 5'-aminoalkyl linkers were dissolved in 20 μL of 0.1 M NaHCO3. EZ-Link NHS-biotin (Pierce, Rockford, USA) (10 μL of a 65 mM solution in dimethyl sulfoxide) was added, and the mixture was incubated at room temperature for 6-16 h in the dark. Then 25 μL of water were added, and the modified oligodeoxynucleotide was separated from the excess of hydrolyzed reagent by two consecutive separations on Micro Bio-Spin 6 columns following the manufacturer's recommendations. After the second spin, the biotinylated oligodeoxynucleotide was precipitated with ethanol-sodium acetate. In control experiments the previously published decoy NF-κB-ODN  was used. In some cases FITC-labeled or biotinylated decoy ODNs were obtained from Sigma-Aldrich. Note that the oligonucleotides used for cell death induction, pull-down assays and whole-cell pull-down assays were similar and could be used interchangeably, except that for pull-down biotinylated oligonucleotides had to be used.
Preparation of liposomes
Liposomes were formulated using a cationic lipid (3β-[N-(N',N',N'-triethylaminopropane)-carbamoyl] cholesterol) iodide (TEAPC-Chol) and neutral colipid dioleoyl phosphatidylethanolamine (DOPE), as previously described . The concentration of cationic lipid was monitored by UV spectroscopy at 226 nm and the value was used to calculate the charge ratio assuming one positive charge for each cationic lipid molecule.
Gel electrophoresis, western blotting
Cells were washed in PBS, lysed in sample buffer (50 mM Tris-HCl pH 6.8 (Bio-Rad, Marnes-la-Coquette, France), 2% sodium dodecyl sulfate (SDS) (Sigma-Aldrich), 20% glycerol (Prolabo, Fontenay-sous-Bois, France), 1 mM sodium vanadate (Na3VO4, Labosi, Elancourt, France), 1 mM dithiothreitritol (DTT) (Merck, Fontenay Sous Bois, France) and 0.01% bromophenol blue (Sigma-Aldrich), sonicated and stored at -70°C. Proteins (50 μg) were separated on SDS-PAGE (10%) and transferred onto nitrocellulose membranes; membranes blocked with 5% dry skimmed milk in TBS were incubated with antibody overnight at 4°C. Anti-phosphotyrosine 705-STAT3 (1/1,000), anti-STAT3 (1/1,000), anti-NF-κB p50 (1/1,000), anti-NF-κB p65 (1/1,000), anti-STAT1 (1/1,000), and anti-OCT1 (1/1,000) were from Cell Signaling, anti-karyopherin/importin α (1:400) was from Santa Cruz (Tebu-bio, Le Perray en Yvelines, France). Blots were washed in TBS-T, incubated with peroxidase-coupled goat anti-mouse (Santa Cruz, Tebu-bio) or goat anti-rabbit (Upstate, Ozyme) secondary antibody (1/20,000) washed in TBS-T and revealed by chemiluminescence (LumiGLO reagent and peroxide; Cell Signaling) and autoradiography (X-Omat R film; Kodak). When necessary, membranes were stripped with Blot Restore Kit (Chemicon International) and reprobed with anti-actin antibody (Cell Signaling). Prestained molecular weight standards (Fermentas, Saint-Rémy-lès-Chevreuse, France) were used. For the quantification analysis, the bands from at least three separate experiments were scanned using a Chemidoc apparatus (Biorad) and quantification performed using the Quantity One software (Biorad). P-values were calculated using a t test.
The TaqMan® Gene Expression Cells-to-CT™ kit (Applied Biosystems, Courtaboeuf, France) was used to extract total RNA and to perform reverse transcription and gene amplification. An Applied Biosystems Custom TaqMan Gene Expression Assay was used; the sequences were chosen to cover exons 5 and 6 to avoid detecting genomic DNA: sense primer: 5'-ccatcttcatcacactcttcctgtt, antisense primer: 5'-accaccgaggagaagatcca, 5'-FAM probe: 5-ctacagtgccaccgtcacc. For the TaqMan Gene Expression Assay (Applied Biosystems) ref. Hs00941525_g1 was used. For cyclophilin A (PPIA), used as a reference, the TaqMan Gene Expression Assay, ref. Hs99999904_m1 was used. All steps were performed following the recommendations of the manufacturer. Relative expression levels of each gene were calculated as previously described.
Cells were grown in 4-well plates to a density of 0.5 106 cells/mL. When the cells reached 50-60% confluence, they were transfected with STAT3-decoy ODN or the hairpin control decoy ODN (2 μg corresponding to 400 nM) in 150 μL of DMEM medium (without SVF) combined with the liposomes (2 μg of cationic lipid). After 6 h at 37°C in a humidified 5% CO2 incubator, the cells were placed in fresh serum-containing medium. Expression was analyzed after 48 h. In other cases, transfection was performed using polyethyleneimine (PEI, Sigma-Aldrich), with an ODN-to-polyethyleneimine ratio of 1:1.
Flow cytometry, cell viability, immunocytochemistry
To measure cell death, cells were resuspended in annexin V-binding buffer, incubated with 5 μL of propidium iodide (BD Pharmingen, Morangis, France) and subjected to flow cytometry analysis, using a BD FACS Canto II Flow Cytometer. Cell viability was also assessed using the trypan-blue exclusion method with a V-cell counter (Beckmann, Villepinte, France).
For immunocytochemistry, cells were grown in 8-well plates (lab-tek, Nunc, Rochester, USA) to a density of 0.5 106 cells/mL. At 50-60% confluence, cells were transfected with the FITC-labeled STAT3-decoy ODN or the FITC-labeled mutated STAT3-decoy ODN. After 48 h the cells were washed in NaCl-phosphate buffer, fixed in 3.7% formaldehyde for 15 mn, permeabilized in 0.1% Triton X-100 for 15 mn and blocked in 5% FCS, 0.1% Tween in NaCl-phosphate buffer for 1 h. Cells were stained with anti-STAT3 antibody (Cell Signaling) (dilution: 1:100) or anti- phosphotyrosine 705-STAT3 antibody (Cell Signaling) (1:100) for 2 h and Alexa Fluor 546-labeled secondary antibody (Invitrogen) (1:200) for 90 mn. After counterstaining with 4', 6'- diamidino-2-phenylindole (DAPI) coverslips were mounted onto glass slides with Vectashield (Vectorlabs, Clinisciences, Montrouge, France). Fluorescence images were acquired using a Zeiss Axioplan2 Deconvolution microscope (Carl Zeiss, Le Pecq, France) and analyzed with Metafer4 (Metasystems, Altlussheim, Germany).
Nuclear protein extracts were obtained as follows: 20 million cells were resuspended in lysis buffer (20 mM Hepes, pH 7.4, 1 mM MgCl2, 10 mM KCl, 0.3% NP40, 0.5 mM dithiothreitol, 0.1 mM EDTA, protease inhibitors: Compete™, Boerhinger) at 4°C for 20 min. The lysates were centrifuged at 14 000 × g for 5 min at 4°C, and the supernatants containing the cytoplasmic proteins were discarded. The pellets were resuspended in the cell lysis buffer adjusted with 20% glycerol and 0.35 M NaCl for 30 min at 4°C. After centrifugation at 14 000 × g for 5 min at 4°C, the supernatants were stored at -80°C. For pull-down assays, 100-200 μg of nuclear protein extracts were incubated for 30 min at 4°C in binding buffer (1% NP40, 50 mM Hepes, pH 7.6, 140 mM NaCl) containing salmon sperm DNA (1 μg/assay) and 1 μg of the biotinylated hairpin decoy ODN or the mutated decoy ODN. The complexes were captured by incubation with 50 μl of avidin-Sepharose beads (neutravidin, Pierce) for 2 h at 4°C. For in-cell decoy ODN pull-down assays, the cells were first transfected with STAT3-decoy ODN or its mutated equivalent, as described under oligonucleotide transfection (see above), and then processed as above by cell lysis and recovery on avidin-Sepharose beads. After extensive washing with binding buffer, complexes were separated on SDS-polyacrylamide (8%) gel, and subjected to immunoblotting using an anti-STAT3 antibody (Cell Signaling). Results were analyzed by chemiluminescence (LumiGLO, Cell Signaling) and autoradiography (X-Omat R, Kodak).
For antibody pull-down assays, 20 million cells were lyzed and resuspended in lysis buffer (20 mM Hepes, pH 7.4, 1 mM MgCl2, 10 mM KCl, 1% NP40, 0.5 mM dithiothreitol, 0.1 mM EDTA, 1 mM orthovanadate, protease inhibitors; Compete™, Boerhinger) at 4°C for 5 min. The lysates were centrifuged at 14 000 × g for 5 min at 4°C, and the supernatants containing the cytoplasmic proteins were either used immediately or stored at -80°C. For the immunoprecipitations, 200 to 400 μg protein was supplemented with albumin-saturated protein G-agarose (Boehringer); after centrifugation (8000 × g, 5 min), the pellet was discarded and the supernatant conserved. Antibody was added (STAT3: 1:100; karyopherin, Santa-Cruz: 1:40) and incubation continued overnight at 4°C. Samples were then supplemented with albumin-saturated protein G-agarose and incubated for 1 h 30 m. The agarose beads were washed three times with TBS and once with TBS-T, and resuspended in SDS-sample buffer. Gel separation and western blotting were performed as described above.
Survival of SW 480 colon carcinoma cells requires activated STAT3
Cytoplasmic sequestration of STAT3 and phospho-STAT3 by STAT3-decoy ODN
Cytoplasmic sequestration of STAT3 and phospho-STAT3 by STAT3-decoy ODN correlates with STAT3 inhibition and cell death
Blockage of STAT3 nuclear transfer by STAT3-decoy ODN is overcome by IL-6-mediated activation of STAT3 or increased expression of recombinant STAT3
STAT3-decoy ODN interferes with the cytoplasmic-nuclear shuttling of STAT3
Inhibition of STAT3 with STAT3-decoy ODN in SW 480 cells is associated with inhibition of NF-κB
Cell death and apoptosis depend partly on STAT1-dependent effector genes [46, 59] (see also  for a review). In the present study, the suppression of STAT1 by RNA silencing prevented cell death induction by STAT3-decoy ODN, indicating a critical role for STAT1, independent of IFNγ activation, in line with previous observations showing STAT1-dependent IFN-independent cell death ; (see also: ). However, STAT1 is the major effector of IFNγ, which is antiproliferative or tumoricidal in several cancer cell types [62, 63]; although STAT3-decoy ODN has been found to induce tumor cell death in several different cell systems [17, 22–24], it can also inhibit STAT1 [17, 64]. Despite their opposing biological effects , STAT1 and STAT3 form heterodimers whose function is unclear . Thus, despite its efficiency in inducing cell death, and although it has been found to have few side effects when administered to primates , suggesting a potential for clinical applications, STAT3-decoy ODN must be optimized so that it can distinguish between STAT3 and STAT1. Work is in progress to try and define the structural constraints that underlie specific recognition of STAT3-decoy ODN by STAT3.
In cancer, STAT3 and NF-κB have been shown to cooperate in promoting cell growth by interacting at different levels of their activating pathways . STAT3 can trap constitutively activated NF-κB within the nucleus of tumor cells . In SW 480 cells, both NF-κB and STAT3 are activated, suggesting a constitutive interleukin secretory loop, as described for several tumor cell systems (see: ). The findings of the present paper indicate that active STAT3 interacts with NF-κB in the colon-carcinoma cell line SW 480, as shown by the presence of NF-κB in STAT3-decoy ODN pull-downs and by reduced NF-κB transcriptional activity. Unphosphorylated STAT3 also interacts with NF-κB, but apparently binds κB sites , and may not be recognized by STAT3-decoy ODN for this reason. Thus, by trapping active STAT3 within the cytoplasm, STAT3-decoy ODN can simultaneously trap the fraction of NF-κB that is associated to active STAT3; this may potentially allow the targeting of a subset of genes that is essential for uncontrolled tumor cell growth.
STAT3-decoy ODN is an efficient inducer of cell death in the colon-carcinoma cell line SW 480. It is shown here to function by trapping activated STAT3 within the cytoplasm by binding to the active dimer and preventing binding to karyopherin, which is required for the transfer of active STAT3 into the nucleus. STAT3-decoy ODN appears to be capable of specifically targeting active STAT3, and not its inactive form. Thus, STAT3-decoy ODN inhibits STAT3 only in cells where STAT3 is activated, such as cancer cells, resulting in cell death without harming healthy cells. Furthermore, the entrapment of STAT3-bound NF-κB adds a new powerful anticancer potential to STAT3-decoy ODN. These results point to the DNA binding domain of STAT3, as well as the process through which activated STAT3 enters the nucleus, as potential sources of active anti-cancer compounds.
We thank Dr. Curt Horvath (Northwestern University, Chicago USA) for the kind gift of DNA-binding mutant STAT3 plasmid, and Dr. David Tabatadze (Zata, Worcester, Mass. USA) for help with oligonucleotide synthesis. IS was supported by a grant from the Association pour la Recherche sur le Cancer (ARC), IN was supported by the Ligue contre le Cancer, and LAK was supported by the Ministère de l'éducation et de la Recherche. This work was funded in part by grants from OSEO (France), the Ligue contre le Cancer (France) and the Ligue contre le Cancer (comité du Val d'Oise).
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