Optimizing experimental radioimmunotherapy investigating the different mechanisms behind radiation induced cell deaths
Sammanfattning: Background. Radiation therapy is an important treatment regimen for malignant disease. Radiation therapy uses ionizing radiation to induce DNA damage in tumor cells in order to kill them. Tumor cells are more sensitive than normal cells, since they have an increased proliferation rate and often lack the ability to properly repair the induced damage. Radiation can be delivered by an external source outside the body, by brachytherapy delivered inside the patient near the tumor, or systemically by injection into the blood stream. When delivered systemically, the radiation is administered as radioisotope alone or conjugated to antibodies targeting tumor antigens (radioimmunotherapy). Radiotherapy (RT) usually is administered using high doses, causing necrotic cell death. Low doses of radiation (by RT or RIT) have been observed to induce different types of cell deaths, like apoptosis, mitotic catastrophe or senescence.Aims. We wanted to elucidate the molecular and cellular events responsible for the induction of cell death in cells of different origin and p53 status. We also wanted to identify the kinetics behind gene expression alterations induced in response to irradiation and correlate these to cell death specific molecular and cellular events. In the end this research aims to identify key regulators of the main radiation induced cell death modalities in order to improve our understanding and potentially use this knowledge to increase treatment efficacy of radiation therapy.Methods. Four different cell lines were used in these studies to elucidate the role of p53 status cell origin in radiation induced cell death. HeLa Hep2 tumor cells have been used previously in our group in several RIT and RT studies. During these studies we observed morphological alterations in shrinking tumors that were typical for mitotic catastrophe. This led to studies on the underlying mechanisms causing these aberrations. Isogenic solid tumor cell lines HCT116 p53 +/+ and HCT116 p53 -/- were included to further elucidate the role of p53, and also to study senescence, one of the main outcomes in irradiated tumor cells. MOLT-4 was finally included to compare these finding to classical apoptosis. Gene expression analysis was done using Illumina bead chip arrays, and pathway analysis was performed using MetaCore (Thomson Reuters).Results. In paper I, II, and III, transient G2/M arrests were observed in HeLa Hep2 and HCT116 p53 -/- cells following irradiation. The lack of p53 in these cells caused checkpoint adaptation due to an unscheduled accumulation of genes promoting mitosis. Anaphase bridges were observedivin HeLa Hep2 cells, as a consequence of premature mitotic entry with unrepaired DNA damage. Centrosome amplification, as well as deregulation of genes involved in centrosome amplification and clustering was observed in both cell lines. We observed changes in expression of several genes responsible for maintaining the spindle assembly checkpoint (SAC) arrest. A prolonged SAC arrest has been shown to be important for execution of mitotic catastrophe. SAC activation was followed by mitotic slippage and a subsequent failure of cytokinesis. We observed multipolar mitoses (both cell lines), multiple- and micronuclei (HeLa Hep2, paper I), and an increased frequency of tetraploid cells (HeLa Hep2 and HCT116 p53 -/- cells). A fraction of HeLa Hep2 cells also displayed apoptotic features, including caspase activation and DNA fragmentation (paper I). These findings indicate that mitotic catastrophe and the activation of a delayed type of apoptosis are involved in cell death following RIT.HCT116 p53 +/+ cells induced both G1 and G2 arrest following irradiation (paper III). Gene expression analysis revealed significantly decreased expression of genes responsible for cell cycle progression (pronounced decrease compared to HeLa Hep2 and HCT116 p53 -/-), especially mitotic genes. The prolonged arrest transitioned into senescence starting 3 days following irradiation and peaked after 7 days. Several genes associated with SASP were upregulated in the same time frame as senescence was induced, further supporting the fact that senescence is the main radiation induced response in HCT116 p53 +/+ cells.MOLT-4 cells, similar to HCT116 p53 +/+ cells, induced both G1 and G2 arrests in response to irradiation (paper IV). Morphological studies revealed apoptotic features like shrunken cells with condensed DNA. Caspase assays showed increased activity of caspases -3, -8, and -9. Gene expression analysis confirmed an increased expression of genes important for both extrinsic (FAS and TRAIL) and intrinsic (BAX) apoptosis. Furthermore, changed expression also included genes involved in cell cycle checkpoints and their regulation and genes important for T-cell activation/proliferation.Conclusions. RIT is successfully used to treat lymphoma, but treatment of solid tumors with RIT is still difficult. This thesis elucidates cellular alterations characteristic for the 3 main radiation death modalities, i.e. mitotic catastrophe, senescence and apoptosis. Furthermore, cell death specific traits are correlated to alterations in gene expression. Treatment efficacy can potentially be improved by finding key cell death mediators to inhibit in combination with radiation.
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