Browsing by Subject "DNS-Reparatur"

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Bedeutung der c-Abl-Aktivität für die Reaktion auf DNA-Schädigung und für die genetische Stabilität Bcr-Abl-negativer Zellen
(2011) Fanta, Silke; Aulitzky, Walter E.
The launch of Imatinib (Glivec®, Gleevec®, STI571) in August 2001 was an important advancement in the therapy of chronic myeloid leukemia (CML). The small-molecule inhibitor directly targets the oncogenic tyrosine kinase Bcr-Abl, which has been identified as the central cause for the development of CML. Treatment with Imatinib is the gold standard in the therapy of CML. However, taking the current state of research, an elimination of the malignant Bcr Abl-positive clone cannot be achieved by treatment with Imatinib. Thus, long-term or even lifelong treatment of patients is necessary. As a consequence, it is of great interest to clarify the biological effects of Imatinib on physiologically normal cells. Previous studies of the group showed that Imatinib treatment of Bcr Abl-positive cells leads to a decreased mutation frequency following DNA damage. Within the scope of the present work, evidence for significantly enhanced mutation rates after DNA damage in non-cancerogenic primary human lymphocytes (PBMC) and murine hematopoietic cell lines (32D and BaF3) after Imatinib treatment was obtained for the first time. Thus, Imatinib treatment of Bcr Abl-negative cells shows opposite effect compared to Bcr Abl-positive cells. It was therefore proven that the Imatinib-related inhibition of Bcr Abl as well as the off-target effects in Bcr Abl-negative cells play an important role in the genetic stability. To determine whether an Imatinib-mediated inhibition of c Abl activity is responsible for effects independent of Bcr Abl, genetic c Abl models were used to assess stress-induced mutation frequency. To this, we employed c Abl-knockout-MEFs (embryonic mouse fibroblasts), which were retransfected with wild type c Abl and a kinase-deficient form, respectively. After DNA damage, there was a significant increase in mutation frequency in the kinase-deficient cells (MEF Abl-KD) when compared to the c-Abl wild type (MEF Abl-wt) cells. Consequently, c-Abl activity is of great importance for the maintenance of genetic stability. Several factors can result in an increased mutation frequency in cells. Examples include altered cell proliferation, impaired DNA repair mechanisms or a delayed induction of cell death. In the latter case, DNA damage is not adequately repaired and passed to the daughter cells. In this study, different hematopoietic cell lines were used to show that neither the pharmacological nor the genetic inhibition of c-Abl activity has an influence on induction of cell death, division rate, cloning efficiency and cell cycle distribution. To investigate how far Imatinib influences the kinetics of DNA strand break repair after irradiation, alkaline comet assays were performed. Imatinib treatment of cells had no influence on induction of strand breaks or constitutive strand breaks prior to irradiation. However, cells treated with Imatinib exhibited a significantly delayed repair of DNA strand breaks. This delay was shown in the same manner in hematopoietic cell line models and in primary human lymphocytes, which were treated with Imatinib as well as with Dasatinib, a second generation Abl-inhibitor. Cell line models with different forms of c-Abl were used to provide evidence that this effect is caused by inhibition of the c-Abl kinase activity. The delayed repair of DNA strand breaks was also seen in cells with a kinase-deficient form of c-Abl (MEF Abl KD). But treatment with Imatinib had no effect on the kinetics of DNA repair in cells that expressed an Imatinib-resistant form of c Abl (c Abl T315I). Double- (DSB) as well as single-strand breaks (SSB) are determined in an alkaline comet assay. By applying neutral conditions, this assay can be modified to exclusively analyze DSB repair. As expected, there was a significantly lower induction of DSB after irradiation when compared to the occurrence of SSB. However, Imatinib did neither influence the induction nor the kinetics of DSB repair. Both pulsed-field gel electrophoresis and the quantification of gamma-H2AX were used to confirm that Imatinib does not affect DSB repair. Rather, the delayed repair kinetics are exclusively caused by an Imatinib-dependent interference with SSB repair. Extensive investigations of the molecular signaling pathways of DNA damage repair show that inhibition of c Abl activity does not affect ATM-Chk2-p53 or ATR-Chk1 signaling. Poly(ADP-ribosyl)ation of proteins is an early event in the processing of the SSB repair. This modification of proteins by addition of long and branched poly(ADP-ribose) chains (PAR) is an essential part of the SSB repair and base excition repair (BER). Both the synthesis and the cleavage of PAR is mediated by the kinases PARP-1 (poly(ADP-ribose) polymerase-1) and PARG (poly(ADP-ribose) glycohydrolase). This activity was determined by quantification of PAR and the percentage of cells, which were PAR-positive at a certain time. Possible effects of an Imatinib-induced inhibtion of c-Abl on poly(ADP-ribosyl)ation were investigated. To this, a method for the measurement of PAR events on a single-cell level was established. Poly(ADP-ribose) residues were marked with a PAR-specific antibody and detection followed by means of a fluorochrome-conjugated secondary antibody. The specificity of the method was proven unequivocally by a complete loss of signal when a specific PARP inhibitor (PJ34) was applied prior to irradiation-induced ribosylation. The advantage of this method is that the simultaneous determination of the DNA content in every cell allows the analysis of ribosylation events in correlation with cell cycle distribution. Based on these experiments it was found that in Imatinib-treated cells both the constitutive and the irradiation-induced poly-ribosylation are significantly enhanced. Furthermore, irradiation does not result in poly-ribosylation of all cells at a certain time: A subpopulation of cells, presumably those in the G0 resting phase, remain PAR-negative before and after irradiation. Thus, a novelty of the work at hand lies in the correlation of ribosylation events and cell cycle distribution before and after DNA damage. In this context, the central role of the Imatinib-mediated inhibition of c-Abl could also be established. The inhibited kinase activity of c-Abl seems to cause a delayed degradation of PAR. This is either caused by decreased activity of the PARP-1 antagonist PARG or by increased activity of PARP-1 itself. A disturbance of the spatially and temporally tightly modulated synthesis and degradation of PAR may lead to a prolonged interaction of PARP-1 with proteins related to SSB repair or BER, e.g. XRCC1 and DNA polymerase beta, thus resulting in the observed delay in DNA damage repair. The present study provides new insights into the impact of Imatinib on Bcr Abl-negative cells. The obtained in vitro data suggest that long-term treatment with c-Abl inhibitors may be associated with an increased likelihood of secondary neoplasias. Despite the outstanding success in Imatinib treatment of CML patients in the chronic phase, the complete elimination of the malignant clone should be the primary goal of the treatment of Bcr-Abl-positive leukemias.
Einfluss eines Glukoseentzugs auf die Strahlenempfindlichkeit von Tumorzellen und Normalzellen
(2018) Ampferl, Rena; Dittmann, Klaus
Radiotherapy is a major pillar of cancer treatment. However, the maximal dose that can be applied to a tumor is limited by side-effects of the irradiated normal tissue. Therefore, to improve treatment success, it is of significant interest to develop new treatment strategies that selectively enhance the cytotoxic effect of radiation in tumor cells while sparing healthy tissue. For this purpose, it is necessary to exploit differences between tumor cells and normal cells. Thus, tumor cells are characterized by metabolizing glucose preferentially to lactate regardless of the availability of oxygen (Warburg effect, aerobic glycolysis), while normal cells oxidize most of the glucose in the mitochondria if oxygen is present. Because the Warburg effect only produces low amounts of ATP per molecule of glucose when compared to mitochondrial glucose oxidation, tumor cells rely on high glucose supply. Hence, it was the aim of this study to investigate whether a glucose starvation during radiotherapy, which requires energy-dependent repair of DNA damage, is an appropriate strategy to selectively enhance radiosensitivity of tumor cells, but not of normal cells. It was shown that glucose starvation inhibited proliferation of the tumor cell lines A549 and FaDu, but not that of the normal fibroblasts HSF7. Moreover, deprivation of glucose induced cell death selectively in tumor cells, which occurred mainly via necrosis. Combining glucose starvation with radiotherapy led to selective radiosensitization of both tumor cell lines, which was accompanied by impaired repair of radiation-induced DNA double-strand breaks (DNA DSBs). In this context, it turned out that in tumor cells glucose is essential for the late stage of DNA DSB repair starting from 13 h after irradiation. Furthermore, an inhibition of radiation-induced histone acetylation as well as KAP1 phosphorylation could be observed in tumor cells following glucose starvation, indicating an impairment of radiation-induced chromatin relaxation. Because opening of the chromatin structure is particularly important for the repair of DNA DSBs within heterochromatin and because these DSBs are the ones that are repaired at late time points after irradiation, it can be assumed that in tumor cells glucose starvation mainly impairs the repair of heterochromatic DNA DSBs. Further investigations revealed that in tumor cells glucose starvation does not cause lack of nuclear acetyl-CoA, which is the substrate for the acetylation of histones, and therefore this could be excluded as cause of the observed inhibition of histone acetylation. However, it is known that the histone deacetylase Sirt1 is activated in response to glucose starvation. Histone deacetylation by Sirt1 could counteract radiation-induced histone acetylation, thus impairing chromatin relaxation as well as repair of DNA DSBs after irradiation. In fact, it was shown that inhibition of Sirt1 by sirtinol can partly abrogate the impaired repair of radiation-induced DNA DSBs that was observed in tumor cells under glucose-free conditions. However, the inhibitory effect of glucose starvation on DNA DSB repair in tumor cells could not only be observed under glucose-free conditions. Thus, reducing the glucose concentration to 0.5 g/l was enough to impair DSB repair following irradiation to the same degree as after total deprivation of glucose. Furthermore, it turned out that under glucose-free conditions DNA DSB repair in tumor cells was promoted by autophagy already after irradiation with 2 Gy. Finally, it was shown that, in addition to DNA DSB repair, also tumor metabolism is influenced by glucose starvation. Thus, deprivation of glucose impaired the radiation-induced switch of glucose metabolism that was characterized by increased aerobic glycolysis and decreased mitochondrial glucose oxidation, and this can also contribute to radiosensitization of the cells. In contrast to tumor cells, glucose starvation neither caused radiosensitization nor impaired the repair of radiation-induced DNA DSBs in normal fibroblasts. Moreover, in these cells, glucose starvation had no influence on histone acetylation and KAP1 phosphorylation after irradiation. These results demonstrate that glucose starvation is an appropriate in vitro strategy to selectively sensitize tumor cells to radiotherapy without influencing the radiosensitivity of normal cells.
Nuclear activation of proteasome in oxidative stress and aging
(2009) Catalgol, Betul; Grune, Tilman
Poly(ADP-ribosyl)ation reactions are of interest in recent years and they take place in DNA repair in different processes especially following oxidative nuclear damage. Proteasomal reactions also take place in repair following oxidative nuclear damage with the degradation of oxidized histones. Antitumor chemotherapy is generally believed to act via the oxidation of nuclear material in the tumor cells. Adaptation to oxidative stress appears to be one element in the development of long-term resistance to many chemotherapeutic drugs. The 20S proteasome has been shown to be largely responsible for the degradation of oxidatively modified proteins in the nucleus. Tumor cells are supposed to have a higher nuclear proteasome activity than do nonmalignant cells. Poly(ADP-ribosyl)ation reactions take place in the tumor cells as a consequence of chemotherapy. Such a reaction might occur with the 20S proteasome ?which is known to increase the activity- and also with histones ?which is firstly shown to decrease the degradation in this study. After hydrogen peroxide treatment of HT22 cells, degradation of the model peptide substrate suc-LLVY-MCA and degradation of oxidized histones in nuclei increased accompanied by an increase in PARP-1 mRNA expression. In the recovery of the level of protein carbonyls, single strand breaks and 8-OHdG, proteasome and PARP-1 were shown to play a role together. This was tested with inhibitor treatments. The proteasomal activation following poly(ADP-ribosyl)ation of proteasome and the decrease in poly(ADP-ribosyl)ation of histones and increase in the proteasomal degradation of histones following H2O2 treatment confirmed our hypothesis. The second part of the thesis shows the changes in PARP-1 and proteasome in different aged fibroblasts with population doublings 19, 36, and 56. The nuclear protective mechanisms were shown to be effected during the senescence process. PARP-1 protein amount decreased whereas there was no change in proteasome amount. PARP activation following H2O2 treatment increased only in young and middle aged cells. In the nuclear extracts of young and old cells, poly(ADP-ribosyl)ation potentials were tested with NAD+ addition into the reaction. In addition to that active proteasome and PARP enzymes were added into the reaction and proteasome activity was measured. With active PARP, proteasome activity was increased both in young and old cells whereas there was no increase in old cells without PARP addition. These results show that proteasome activation is mainly limited by PARP activity. Taken together all results demonstrate the importance of PARP mediated proteasome activation in the repair of oxidatively damaged chromatin.
Untersuchungen zum molekularen Wirkmechanismus des Radioprotektors O-Phospho L-Tyrosin : Wechselwirkungen von Phosphotyrosin mit Aktivierungsprozessen des epidermalen Wachstumsfaktorrezeptors
(2008) Wanner, Gabriele; Rodemann, H.-Peter
Summary Cancer is, after cardiovascular diseases, the main cause of death in Germany. The five-year-survival-rate averages at 35% to 45% and is supported by tumour treatment with ionizing irradiation. Radiotherapy is among surgical interventions and chemotherapeutical methods one of the most important treatment procedure for oncological diseases. In spite of technical improvements and modern therapy designs the ineluctable damage of the tumour surrounding normal tissue acts dose limiting and may lead to acute and late normal tissue reactions and associated side effects. Radioprotectors are applied to protect healthy tissue in the radiation field, but should not protect tumour tissue against radiation effects. The potential dose escalation due to enhanced radiation-tolerance would be associated with increased tumour control. The intention of this work was to shed light on the molecular mode of action of the radioprotector O-Phospho L-Tyrosine (pTyr) and the associated modulation of radiation-induced effects on signalling cascades. The data presented provide evidence that preincubation of human fibroblasts with pTyr leads to a significant increase of cell survival after irradiation. The pTyr-mediated radioprotection was found to be dependent on the availability of the tumour suppressor TP53. Only cells characterized by a wildtype TP53, but not cells with mutated TP53 cells were protected by a pTyr-treatment. Given that a large proportion of human tumours express a mutated TP53, more than 50% of tumours are considered to be treated with pTyr and irradiation in combination. Preliminary work on the field of pTyr-induced molecular mechanism showed an interaction between pTyr and the EGFR-associated signalling pathway, which positively influences radiation-induced DNA-repair (Dittmann et al., JBC 2005). Based on these data we investigate the influence of pTyr treatment on a molecular level. The following results were obtained: 1.pTyr treatment stimulates accumulation of the EGFR in the nucleus, which is involved in regulation of DNA-PK-dependent DNA-repair (NHEJ). 2.pTyr-mediated stimulation of DSB-DNA-repair processes is dependent on functional tumour suppressor TP53. 3.Ionizing radiation and pTyr are able to modulate the phosphorylation status of nuclear EGFR at the position No. 654. This phosphorylation site plays an important role in accumulation of EGFR in the nucleus. 4.The isoform ε of the protein kinase C family is responsible for the pTyr- and radiation-induced T654 phosphorylation of the EGFR in the nucleus. 5.The kinase activity of the nuclear PKCε is essential for nuclear EGFR accumulation and EGFR-associated stimulation of the DNA-PK-dependent DNA-repair. 6.The considered second messenger in the pTyr- and radiation-induced activation of PKCε is diacylglycerol, which is limited by its regulatory kinase DGK θ. 7. Diacylglycerol is generated by hydrolyzation of phosphoinositol biphosphate, which can be stimulated by pTyr treatment and ionizing irradiation. 8.pTyr enhances complex formation between nuclear EGFR, phosphorylated form of DNA-PK and DNA after irradiation. Aim of this work was to elucidate molecular vertices of a pTyr treatment in order to promote the clinical application of pTyr in the context of radiooncological therapy. From present data we suggest that combined treatment of TP53-mutated tumours with pTyr and irradiation improves survival of tumour surrounding normal tissue by influencing DNA-repair processes positively and therefore takes advantage in radiotherapy-treatment. Furthermore this work gives a better insight into radiation-mediated and radioprotective signalling pathways and helps to achieve a deeper understanding in molecular mechanisms after irradiation and radiation-associated survival signals.