Mechanisms of Antitumor Activity of Low Doses of Radiation Associated with Activation of Cells' Defense System.

Background: The effects of ionizing radiation (IR) involve a highly orchestrated series of events in cells, including DNA damage and repair, cell death, and changes in the level of proliferation associated with the stage of the cell cycle. A large number of existing studies in literature have examined the activity of genes and their regulators in mammalian cells in response to high doses of ionizing radiation. Although there are many studies, the research in effect of low doses of ionizing radiation remains limited. Though much progress has been made in understanding the basic principles of effects of low doses of radiation on individual components of biological systems, less is known about how low doses affect target molecules and regulate the cellular networks (e.g., activation of the immune system, genes and their regulators in the phenomenon of hormesis, and the formation of an adaptive response). These observations determined the purpose of the work: to investigate the activity of genes and non-coding RNAs (long non-coding RNAs and microRNAs) in various organs of mice with transplanted Lewis carcinoma after low-dose radiation.

Materials and methods: Twenty-four female C57Bl/6 mice were transplanted subcutaneously with Lewis carcinoma cells (10⁵ cells in 0.2 mL of Hanks' solution). Total 4-fold X-ray irradiation with an interval of 4 days at a dose of 0.075 Gy (0.85 Gy/min) was performed on the RUST M1 from 6 days after transplantation; the tumor size was measured daily. The mice were divided into the following groups: "Biocontrol", "Biocontrol + irradiation", "Tumor" and "Tumor + irradiation". On the 19th day from the beginning of the experiment, the mice were euthanized. The expression profiles of mRNA genes, long non-coding RNAs, and microRNAs controlling the response to radiation were determined in the bone marrow, thymus, spleen, and tumor of mice.

Results: Fractionated low-dose irradiation of mice with transplanted Lewis carcinoma caused a growth decrease of implanted tumor cells compared to the similar group without irradiation. At the same time, there was an activation of oncosuppressors and a decrease in the activity of oncogenes in the thymus and spleen of mice with tumor and irradiation. In the "Tumor" group, without irradiation, the number of activated oncogenes prevailed over the number of inactivated ones.

Conclusions: Thus, the low-dose radiation exposure led to the activation of antitumor immunity in mice, which manifested itself in slowing tumor growth in animals and the induction of oncosuppressors and inhibition of oncogene expression.