Low-dose radiation induces adaptive response in normal cells, but not in tumor cells: in vitro and in vivo studies

Biological effects of low-dose radiation (LDR) are distinguishable from those of high-dose radiation. Hormetic and adaptive responses are such two examples. However, whether adaptive response could be induced in tumor cells by LDR, especially under in vivo condition, remains elusive, and was systemically investigated in the present study. Four tumor cell lines: two human leukemia cell lines (erythroleukemia cell line K562, and acute promyelocytic leukemia cell line HL60), and two human solid tumor cell lines (lung carcinoma cell line NCI-H446 and glioma cell line U251), along with one normal cell line (human fibroblast cells, MRC-5), were irradiated with LDR at 75 mGy of X-rays as D1 and then 4 Gy of X-rays as D2 (i.e.: D1 + D2) or only 4 Gy of X-rays (D2 alone). Three tumor-bearing animal models were also used to further define whether LDR induces adaptive response in tumor cells in vivo. Adaptive response was observed only in normal cell line, but not in four tumor cell lines, in response to LDR, showing a resistance to subsequent D2-induced cell growth inhibition. Three tumor-bearing mouse models with U251, NCI-H446 or S180 tumor cells were used to confirm that pre-exposure of tumor-bearing mice to D1 did not induce the resistance of tumor cells in vivo to D2-induced tumor growth inhibition. Furthermore, a higher apoptotic effect, along with higher expression of apoptosis-related genes P53 and Bax and lower expression of anti-apoptosis gene Bcl-2, was found in tumor cells of the tumor-bearing mice exposed to D1 + D2 than those in the tumor cells of the tumor-bearing mice exposed to D2 alone. These results suggest that LDR does not induce adaptive response in the tumor cells under both in vitro and in vivo conditions, which is a very important, clinic-relevant phenomenon.