Radiosensitivity and Bystander Response in X-Ray-Irradiated Tumour and Normal Epithelial Cells of Breast and Prostate Origin.

Introduction: Radiotherapy (RT) outcome is governed by radiosensitivity and DNA repair capacity of tumour cells and their interaction with surrounding normal tissues and vice versa. As radiosensitivity varies with the origin and genetic makeup of the cell, this study compares direct and bystander responses in directly targeted to X-rays and non-targeted (bystander) tumour (MCF-7 and PC3) and normal (MCF10A and HPrEC) epithelial cells of breast and prostate origin.

Methods: Cells were exposed to X-rays (0, 2, and 4 Gy) by a clinical linear accelerator and co-cultured with corresponding unirradiated cells. Micronucleus (MN) and clonogenic assays were adopted to quantify the DNA damage and survival fraction (SF), respectively, in all cells and multicolour fluorescence in situ hybridization (m-FISH) in MCF-7 and PC3 cells to identify the chromosomes frequently involved in translocations.

Results: Directly targeted tumour and normal cells showed a significant increase in MN frequency and decrease in SF. MN frequency increased from 0.023 ± 0.004 (control) to 0.076 ± 0.008 (2 Gy) and 0.177 ± 0.013 (4 Gy) in MCF-7 cells. MCF10A showed MN frequency of 0.049 ± 0.007 (control), 0.128 ± 0.011 (2 Gy), and 0.219 ± 0.014 (4 Gy). SF was significantly higher in MCF-7 (0.39 ± 0.03 and 0.15 ± 0.02) cells than MCF10A (0.30 ± 0.02 and 0.12 ± 0.01). MN frequency in PC3 cells increased from 0.056 ± 0.007 (control) to 0.168 ± 0.012 (2 Gy) and 0.378 ± 0.019 (4 Gy). HPrEC exhibited MN frequency of 0.018 ± 0.004 (control), 0.058 ± 0.007 (2 Gy), and 0.147 ± 0.012 (4 Gy). SF was higher in HPrEC (0.72 ± 0.03 and 0.40 ± 0.02) cells than PC3 (0.22 ± 0.01 and 0.09 ± 0.004). Similarly, a significant increase in MN frequency was observed in the non-targeted cells when compared to that of control, confirming occurrence of radiation-induced bystander effect. Thus, the results indicate radiation sensitivity differs among the cell types. The m-FISH results reveal a non-random distribution of X-irradiation-induced breaks and translocation. In directly targeted cells, chromosomes 7, 16, 17, 20, 21, 22 (MCF-7) and 3, 4, 6, 14, 17 (PC3) showed frequent involvement in translocations. Chromosomes 17 and 20 (MCF-7) and 10, 11, and 18 (PC3) were frequently involved in non-targeted cells.

Conclusion: The present study results indicate that the tumour cells demonstrated higher radiosensitivity and a stronger bystander response than normal cells. Intrinsic molecular factors and genome organization affect both targeted and non-targeted responses, emphasizing their relevance for optimizing RT strategies.