Introduction:
Hormesis and adaptive responses are two important biological effects of low-dose ionizing radiation (LDIR) in organism and mammalian cell lines. Notably, LDIR generates distinct
biological effects in cancer cells from normal cells, e.g., it may affect the growth of cancer cells via the activation of certain cell signaling pathway, which does not exist in normal cells. Therefore, LDIR is considered as a promising assistant method of clinical cancer therapy.
Materials and Methods:
In order to investigate the intermittent LDIR effect on breast cancer, MDA–MB–231 cells were divided into four experimental groups. Group A, cells were irradiated in 10 fractions
with a dose of 30 mGy at each one. The time interval between two irradiations was 24h.
Group B, were cultured simultaneously for the same10 days and received a dose of 300 mGy
and subsequently a high dose of 2 Gy after 24h. Group C, accepted a single high dose of 2 Gy.
Group D was mock irradiation group as control. We examined cell proliferation activity using
MTT assay and PI-Annexin V kit to assay cell apoptosis after 24 hours of last irradiation dose
(2Gy). Also we used RT-PCR to examine the cell cycle arrest and apoptosis pathway. For this
purpose we examined p21 and cespase3 genes in this study.
Results:
The intermittent LDIR increased cell apoptosis up to 32.55% and in other groups, apoptosis
were 30.14%, 25.49% and 8.37% in single LDIR, HDIR and control group respectively. The
apoptosis percentages were markedly higher than in the intermittent LDIR groups (P<0.01).
Through the cell growth assays, we observed that 300 mGy intermittent LDIR significantly
increased the killing effect of radiotherapy (viability: 71.95%) (P<0.01). Although apoptosis
in intermittent LDIR was the highest (32.55%) in comparison with other groups, the
expression of caspase3 gene in this group was the lowest (1.42fold), 4.26fold and 5.2 fold in
single LDIR and HDIR respectively. On the other, the expression of p21 gene, which plays the
role of cell cycle arresting, was the lowest in intermittent LDIR group (1.21fold) which
suggests that low dose of radiation can reduce the expression of p21 gene and promotes cell
cycle in damaged cell and finally causes the death of cancer cells.
Conclusion:
Intermittent LDIR followed by HDIR are a novel strategy to improve radiotherapy efficiency
and could be combined with other therapeutic modalities for cancer treatments. Low dose
has a different mechanism for killing cancer cells, therefore, it has no killing effect
on normal cells.