In this study, we demonstrated that the supplementation of medium with 1 mM Ca2+, Mg2+ and Zn2+ shortly before and after irradiation affected cell radiation response differently. Ca2+ and Mg2+ could inhibit the proliferation and promote the apoptosis after DNA injury in irradiated lung cancer cells. On the contrary, Zn2+ could inhibit cell apoptosis and suppressing cell proliferation induced by irradiation. The DNA DSB damage and DNA repair seem to play an important role in regulating the divalent cations mediated radiation response.
The BRCA1 and KU70 were involved in homologous recombination (HR) and non-homologous endjoining (NHEJ) pathways respectively, both of which play important roles in repair mechanism of DSB generated by ionizing radiation (20, 21). Liu et al reported that the involvement of calcium/calmodulin in regulating the Ku protein expression(22). Moreover, the magnesium and the zinc was also reported to be essential for DNA double-strand break repair by DNA-dependent protein kinase (DNA-PK) (23, 24). In this research, it was further verified that Ca2+ and Mg2+ could block two DSB repair pathways of BRCA1 and Ku70, inhibit phosphorylating survival promoter ERK, activating apoptotic regulator p53 induced by irradiation. Interestingly, Zn2+ showed an opposite effect, which was consistent with the observed radiation effect above. Accordingly, these protein variations might be the underlying molecular mechanisms of radiation effect regulated by different divalent cations.
However, the degree of radiation effect was not equally in the two lung cancer cells. Specifically, compared with that in A549 cells, the NCI-H446 cells with more radiosensitivity (25) showed a remarkably radiosensitization by Ca2+ and Mg2+ treatments. So whether the combination effect of divalent cations and irradiation has a relationship with the radiosensitivity of the cells or the cellular properties still need to be further elaborated.
Our research was consistent with previous reports suggested that Mg2+ can act as radiosensitizer in the absence of oxygen (26). Actually, it has been found for a long time that the endonuclease, the enzyme participates in the DNA degradation, was responsible for programmed cell death or apoptosis in a Ca2+/Mg2+ dependent manner partially (27). This nuclease activity could also be inhibited by Zn2+, which was shown to block apoptosis (28). Moreover, the concentrations of the Ca2+, Mg2+ and Zn2+ could activate or inhibit the purified endonuclease and then affect the process of apoptosis (29). On the basis of these results, we speculated that the different radiation effects mediated by the divalent cations might have a relationship with the endonucleas activity variation under the cations shift. Thamilselvan et al (30) found that the concentrations of the divalent cations affect the radiation effect differently.
On the other hand, newly research found that the cell-cell communication between individual cells in a cellular population plays an important role in radiation-induced responses (31), and the cell-cell interaction requires the presence of divalent cations for better stability (32). It was found that Ca2+ signaling may mediate the cell communication induced by irradiation to lead to the chromosome damage (33). It hints that divalent cations might influence the cell communication between the cells and had a potential role in adjusting the degree of radiation response.
In current study we observed all the divalent cations could down-regulate p38 activation and cell invasion induced by irradiation. Previous studies showed that 2.5–10 mM Ca2+ and 1–10 mM Zn2+ could alter the ovarian cancer cell adhesion, invasion and metastasis (10), probably through the cells binding to extracellular matrix proteins and malignant phenotype modulation mediated by integrin (11, 34). We further improved the role of divalent cations in the cancer phenotype change under the radiation exposure.
In summary, our study provided novel evidence that the potentials of Ca2+ and Mg2+ in combination with irradiation influence the effects induced by irradiation, which inhibit cell invasion, enhance DNA damage, suppress cell proliferation and block apoptosis. Furthermore, the clinical trials will be needed to address not only the reasonable concentrations of the divalent cations used, but also the efficacy and adverse effects of the combination of radiotherapy and divalent cations.