Short and long term radiation effects after ionizing radiation and its dependence on chromatin modification and repair inhibition in human lung cell lines

The dynamic nature of chromatin and its modifications play a critical role in transcriptional gene regulation and thus can affect several cellular functions such as differentiation, replication, DNA damage response and activation of cell death. Alterations in cellular epigenetic and genetic structure after IR can lead to long term effects, such as survival and genomic instability. Additionally, DSB repair is thought to be differentially regulated in euchromatin and heterochromatin. Histone methylation is an important modification related to the compactness and transcriptional activity of chromatin. Thus histone methyltransferases (HMT) play a pivotal role in determining the functional status of chromatin. The human HMT SUV39h1 and SUV39h2 are altered in various types of human cancers. Here we show that the knockout Suv39h1/2 mice fibroblasts (Fbs) have significantly lower clonogenic survival, which is attributed to reduced DSB repair capacity of these cells. The downregulation of HMT SUV39H1 in human lung cancer cell lines leads to significantly reduced number of radiation-induced residual γH2AX. However, the effect of SUV39H1 downregulation on clonogenical survival was less marked as compared to the mouse knockout model. Additionally, the HMT inhibitor Chaetocin, significantly reduces the radiation dose necessary to control 50% of a plaque-monolayer culture after treatment with low concentrations, reduces the cell proliferation and significantly increased apoptosis in lung cancer cell lines. Moreover Chaetocin was found to remodel the nuclear chromatin structure by forming chaetocin-induced chromatin condensation/clustering which is mainly associated with heterochromatin domains in primary human Fbs, transformed mFbs and in the tumour cell line H1299

DNA repair inhibitors sensitize cells differently to high and low LET radiation

The aim of this study was to investigate effects of high LET α-radiation in combination with inhibitors of DDR (DNA-PK and ATM) and to compare the effect with the radiosensitizing effect of low LET X-ray radiation. The various cell lines were irradiated with α-radiation and with X-ray. Clonogenic survival, the formation of micronuclei and cell cycle distribution were studied after combining of radiation with DDR inhibitors. The inhibitors sensitized different cancer cell lines to radiation. DNA-PKi affected survival rates in combination with α-radiation in selected cell lines. The sensitization enhancement ratios were in the range of 1.6–1.85 in cancer cells. ATMi sensitized H460 cells and significantly increased the micronucleus frequency for both radiation qualities. ATMi in combination with α-radiation reduced survival of HEK293. A significantly elicited cell cycle arrest in G2/M phase after co-treatment of ATMi with α-radiation and X-ray. The most prominent treatment effect was observed in the HEK293 by combining α-radiation and inhibitions. ATMi preferentially sensitized cancer cells and normal HEK293 cells to α-radiation. DNA-PKi and ATMi can sensitize cancer cells to X-ray, but the effectiveness was dependent on cancer cells itself. α-radiation reduced proliferation in primary fibroblast without G2/M arrest