Bernard Salles

Cancer-Associated Adipocytes Exhibit an Activated Phenotype and Contribute to Breast Cancer Invasion

Early local tumor invasion in breast cancer results in a likely encounter between cancer cells and mature adipocytes, but the role of these fat cells in tumor progression remains unclear. We show that murine and human tumor cells cocultivated with mature adipocytes exhibit increased invasive capacities in vitro and in vivo, using an original two-dimensional coculture system. Likewise, adipocytes cultivated with cancer cells also exhibit an altered phenotype in terms of delipidation and decreased adipocyte markers associated with the occurrence of an activated state characterized by overexpression of proteases, including matrix metalloproteinase-11, and proinflammatory cytokines [interleukin (IL)-6, IL-1β]. In the case of IL-6, we show that it plays a key role in the acquired proinvasive effect by tumor cells. Equally important, we confirm the presence of these modified adipocytes in human breast tumors by immunohistochemistry and quantitative PCR. Interestingly, the tumors of larger size and/or with lymph nodes involvement exhibit the higher levels of IL-6 in tumor surrounding adipocytes. Collectively, all our data provide in vitro and in vivo evidence that (i) invasive cancer cells dramatically impact surrounding adipocytes; (ii) peritumoral adipocytes exhibit a modified phenotype and specific biological features sufficient to be named cancer-associated adipocytes (CAA); and (iii) CAAs modify the cancer cell characteristics/phenotype leading to a more aggressive behavior. Our results strongly support the innovative concept that adipocytes participate in a highly complex vicious cycle orchestrated by cancer cells to promote tumor progression that might be amplified in obese patients.

The Loss of γH2AX Signal is a Marker of DNA Double Strand Breaks Repair Only at Low Levels of DNA Damage

The induction of DNA double-strand breaks (DSBs) by genotoxic treatment leads to hightoxicity and genetic instability. Various approaches have been undertaken to quantify thenumber of breaks and to follow the kinetic of DSB repair. Recently, the phosphorylation ofthe variant histone H2AX (named γH2AX), quantified by specific immunodetectionapproaches, has provided a valuable and highly sensitive method to monitor DSBs formation.Although it is admitted that the number of γH2AX foci reflected that of DSBs, contradictoryreports leave open the question of a link between the disappearance of γH2AX signal andDSBs repair. We determined γH2AX expression (i) in cells either proficient or not in DSBsrepair capacity, (ii) after exposure to ionizing radiation (IR) or calicheamicin γ1, aradiomimetic compound, (iii) and by three different immunodetection methods, focinumbering, flow cytometry or Western blotting. We showed here that γH2AX loss correlateswith DSB repair activity only at low cytotoxic doses, when less than 100-150 DSBs breaksper genome are produced, independently of the method used. In addition, in DNA repairproficient cells, the early decrease in the number and intensity of γH2AX foci observed after a2 Gy exposure was not associated with a significant change in the global γH2AX level asdetermined by Western blotting or flow cytometry. These results suggest that thedephosphorylation step of γH2AX may be limiting and that the loss of foci is mediated notonly by γH2AX dephosphorylation but also through its redistribution towards the chromatin.

Interplay between Ku, Artemis, and the DNA-dependent protein kinase catalytic subunit at DNA ends.

Repair of DNA double strand breaks (DSB) by the nonhomologous end-joining pathway in mammals requires at least seven proteins involved in a simplified two-step process: (i) recognition and synapsis of the DNA ends dependent on the DNA-dependent protein kinase (DNA-PK) formed by the Ku70/Ku80 heterodimer and the catalytic subunit DNA-PKcs in association with Artemis; (ii) ligation dependent on the DNA ligase IV·XRCC4·Cernunnos-XLF complex. The Artemis protein exhibits exonuclease and endonuclease activities that are believed to be involved in the processing of a subclass of DSB. Here, we have analyzed the interactions of Artemis and nonhomologous end-joining pathway proteins both in a context of human nuclear cell extracts and in cells. DSB-inducing agents specifically elicit the mobilization of Artemis to damaged chromatin together with DNA-PK and XRCC4/ligase IV proteins. DNA-PKcs is necessary for the loading of Artemis on damaged DNA and is the main kinase that phosphorylates Artemis in cells damaged with highly efficient DSB producers. Under kinase-preventive conditions, both in vitro and in cells, Ku-mediated assembly of DNA-PK on DNA ends is responsible for a dissociation of the DNA-PKcs·Artemis complex. Conversely, DNA-PKcs kinase activity prevents Artemis dissociation from the DNA-PK·DNA complex. Altogether, our data allow us to propose a model in which a DNA-PKcs-mediated phosphorylation is necessary both to activate Artemis endonuclease activity and to maintain its association with the DNA end site. This tight functional coupling between the activation of both DNA-PKcs and Artemis may avoid improper processing of DNA.

The double life of the Ku protein : Facing the DNA breaks and the extracellular environment

The Ku heterodimer (Ku70/Ku80) plays a central role in DNA double strand break recognitionand repair. It has been shown, more than ten years ago, that Ku is also expressed at the cellsurface of different cells types along with its intra-cellular pool within the nucleus and thecytoplasm but involvement of Ku in cell-cell and cell-extracellular matrix adhesion has beenonly recently demonstrated. In addition, we have shown that Ku may have a second andunexpected activity in cell/microenvironment interaction. Indeed, Ku appears to be involved inextra-cellular proteolytic processes through its specific interaction, on the cell surface, with thematrix metalloprotease 9. Taken together, these results suggest that Ku function at the cellsurface is likely to be important in tumour invasion. Various fundamental questions arise fromthese observations. How Ku is expressed on the cell surface, why a protein with completelyunrelated functions also serve as an « integrin like »molecule once expressed at the cell surfaceand is this functional moonlighting of Ku related to cell transformation remain open issues thatwill be discussed here.

The radioprotective effect of the 24 kDa FGF-2 isoform in HeLa cells is related to an increased expression and activity of the DNA dependent protein kinase (DNA-PK) catalytic subunit.

We previously reported that overexpression of the 24u2009kDa basic fibroblast factor (or FGF-2) isoform provides protection from the cytotoxic effect of ionizing radiation (IR). DNA double-strand breaks (DSB), the IR-induced lethal lesions, are mainly repaired in human cells by non-homologous end joining system (NHEJ). NHEJ reaction is dependent on the DNA-PK holoenzyme (composed of a regulatory sub-unit, Ku, and a catalytic sub-unit, DNA-PKcs) that assembles at sites of DNA damage. We demonstrated here that the activity of DNA-PK was increased by twofold in two independent radioresistant cell lines, HeLa 3A and CAPAN A3, overexpressing the 24u2009kDa FGF-2. This increase was associated with an overexpression of the DNA-PKcs without modification of Ku expression or activity. This overexpression was due to an up-regulation of the DNA-PKcs gene transcription by the 24u2009kDa FGF-2 isoform. Finally, HeLa 3A cells exhibited the hallmarks of phenotypic changes associated with the overexpression of an active DNA-PKcs. Indeed, a faster repair rate of DSB and sensitization to IR by wortmannin was observed in these cells. Our results represent the characterization of a new mechanism of control of DNA repair and radioresistance in human tumor cells dependent on the overproduction of the 24u2009kDa FGF-2 isoform.

A DNA-dependent stress response involving DNA-PK occurs in hypoxic cells and contributes to cellular adaptation to hypoxia

DNA-dependent protein kinase (DNA-PK) is involved in DNA double-strand break (DSB) signalling and repair. We report that DNA-PK is activated by mild hypoxia conditions (0.1–1% O2) as shown by (1) its autophosphorylation on Ser2056, and (2) its mobilisation from a soluble nucleoplasmic compartment to a less extractable nuclear fraction. The recruitment of DNA-PK was not followed by activation and recruitment of the XRCC4–DNA-ligase-IV complex, suggesting that DSBs are not responsible for activation of DNA-PK. To unravel the mechanism of DNA-PK activation, we show that exposure of cells to trichostatin A, a histone deacetylase inhibitor, leads to DNA-PK autophosphorylation and relocalisation to DNA. Histone acetylation (mainly H3K14) is increased in hypoxic cells and treatment with anacardic acid, an inhibitor of histone acetyl transferase, prevented both histone modifications and DNA-PK activation in hypoxic conditions. Importantly, in using either silenced DNA-PK cells or cells exposed to a specific DNA-PK inhibitor (NU7026), we demonstrated that hypoxic DNA-PK activation positively regulates the key transcription factor HIF-1 and one subsequent target gene, GLUT1. Our results show that hypoxia initiates chromatin modification and consequently DNA-PK activation, which positively regulate cellular oxygen-sensing and oxygen-signalling pathways.

Transfer of Ku86 RNA antisense decreases the radioresistance of human fibroblasts.

Ku86 has been shown to be involved in DNA double-strand break (DSB) repair and radiosensitivity in rodents, but its role in human cells is still under investigation. The purpose of this study was to evaluate the radiosensitivity and DSB repair after transfection of a Ku86-antisense in a human fibroblast cell line. Simian virus 40-transformed MRC5V1 human fibroblasts were transfected with a vector (pcDNA3) containing a Ku86-antisense cDNA. The main endpoints were Ku86 protein level, Ku DNA end-binding and DNA protein kinase activity, clonogenic survival, and DSB repair kinetics. After transfection of the Ku86-antisense, decreased Ku86 protein expression, Ku DNA end-binding activity, and DNA protein kinase activity were observed in the uncloned cellular population. The fibroblasts transfected with the Ku86-antisense showed also a radiosensitive phenotype, with a surviving fraction at 2 Gy of 0.29 compared with 0.75 for the control and 20% of unrepaired DSB observed at 24 hours after irradiation compared with 0% for the control. Several clones were also isolated with a decreased level of Ku86 protein, a surviving fraction at 2 Gy between 0.05 and 0.40, and 10–20% of unrepaired DSB at 24 hours. This study is the first to show the implication of Ku86 in DSB repair and in the radiosensitivity of human cells. This investigation strongly suggests that Ku86 could constitute an appealing target for combining gene therapy and radiation therapy.

Loss of ATM positively regulates the expression of hypoxia inducible factor 1 (HIF-1) through oxidative stress: Role in the physiopathology of the disease.

Ataxia Telangiectasia (AT) is an autosomal recessive disorder characterized by a wide variety of progressive clinical symptoms. This includes neuronal degeneration, oculocutaneous telangiectasias, diabetes mellitus, immunodeficiency, increased risk of cancer and sensitivity to ionizing radiation. The gene mutated in this disease, ATM (Ataxia Telangiectasia Mutated), encodes a protein kinase involved in DNA double strand breaks signalling and repair. ATM deficient cells also display an increase in oxidative stress, by poorly characterized mechanism(s), which clearly contributes to the neurodegenerative aspect of the disease. Despite these advances, the occurrence of the vascular abnormalities, glucose intolerance and insulin resistance remains poorly understood. In different cellular models where ATM expression was disrupted, we demonstrated that the absence of ATM leads to an increased expression of both subunits of the transcription factor Hypoxia Inducible Factor 1 (HIF-1). We also observed enhanced trans-activating functions of HIF-1. HIF-1 is the central regulator of responses to hypoxia which induces the transcription of genes involved in angiogenesis (e.g VEGF -Vascular Endothelial Growth Factor) and cellular metabolism (e.g GLUT-1). Interestingly, we demonstrated that ATM disruption positively regulates both expression and function of the basal glucose transporter GLUT-1 as well as the proangiogenic factor, VEGF. In addition, our results suggest that the absence of ATM increases HIF-1 proteins biosynthesis, and this effect is dependant on the oxidative stress existing in ATM deficient cells. Our compelling results highlight a new link between ATM deficiency and the clinical features of the disease and provide a molecular link between ATM down-regulation and the increase in tumor angiogenesis observed in human breast cancers.

Cell-surface MMP-9 regulates the invasive capacity of leukemia blast cells with monocytic features.

The metalloprotease 9 (MMP-9), a known mediator of tumour invasion, is secreted as a 92kDa pro-form but a non-secreted variant of 85 Kda has been described. The importance of this variant pro-form in tumour progression remains poorly defined. We previously showed that the DNA repair protein Ku interacts at the cell surface of leukaemia cell lines with the 85 Kda pro-form of MMP-9 and these Ku/MMP-9 complexes regulates cell invasion, highlighting their importance in haematological malignancies. We demonstrate here that all samples of acute myeloid leukaemia (AML) blasts purified from bone marrow of 16 affected patients express the 85 Kda form of MMP-9. However, only AML that display monocytic lineage markers (AML4/5) express this form at the cell surface with co-expression of the membrane associated form of Ku. Blocking antibodies directed against Ku or MMP-9 specifically inhibited cell invasion of those expressing Ku/MMP-9 on the cell surface. The membrane form of Ku might represent an important factor in the exposition to the cell surface of this specific MMP-9 pro-form in AML with monocytic features. These results might have important functional significance in the occurrence of extra-medullar infiltrates of leukaemia cells that occurs frequently during the onset of monocyte-related AML sub-types.

Cell nonhomologous end joining capacity controls SAF-A phosphorylation by DNA-PK in response to DNA double-strand breaks inducers

Aiming to identify novel phosphorylation sites in response to DNA double-strand breaks (DSB) inducers, we have isolated a phosphorylation site on KU70. Unexpectedly, a rabbit antiserum raised against this site cross-reacted with a 120 kDa protein in cells treated by DNA DSB inducers. We identified this protein as SAF-A/hnRNP U, an abundant and essential nuclear protein containing regions binding DNA or RNA. The phosphorylation site was mapped at S59 position in a sequence context favoring a S-hydrophobic consensus model for DNA-PK phosphorylation site in vivo. This site was exclusively phosphorylated by DNA-PK in response to DNA DSB inducers. In addition, the extent and duration of this phosphorylation was in inverse correlation with the capacity of the cells to repair DSB by nonhomologous end joining. These results bring a new link between the hnRNP family and the DNA damage response. Additionaly, the mapped phospho-site on SAF-A might serve as a potential bio-marker for DNA-PK activity in academic studies and clinical analyses of DNA-PK activators or inhibitors.