Abstract
In daily life, humans are exposed to the extremely low-frequency electromagnetic fields (ELF-EMFs) generated by electric appliances, and public concern is increasing regarding the biological effects of such exposure. Numerous studies have yielded inconsistent results regarding the biological effects of ELF-EMF exposure. This study showed that ELF-EMFs activate the ATM-Chk2-p21 pathway in HaCaT cells to decrease cell proliferation. To present well-founded results, we comprehensively evaluated the biological effects of ELF-EMFs at the transcriptional, protein, and cellular levels. Human HaCaT cells from an immortalized epidermal keratinocyte cell line were exposed to a 1.5 mT, 60 Hz ELF-EMF for 144 h. The ELF-EMF caused G1 arrest and decreased colony formation. Protein expression experiments revealed that ELF-EMFs induced the activation of the ATM/Chk2 signaling cascades. In addition, the p21 protein, a regulator of cell cycle progression at G1 and G2/M, exhibited a higher level of expression in exposed HaCaT cells compared with the expression of sham-exposed cells. The ELF-EMF-induced G1 arrest was diminished when the CHK2 gene expression (which encodes checkpoint kinase 2; Chk2) was suppressed by specific small interfering RNA (siRNA). These findings indicated that ELF-EMFs activated the ATM-Chk2-p21 pathway in HaCaT cells resulting in cell cycle arrest at the G1 phase. According to the results of HaCaT cells, it is natural to suspect whether ELF-EMFs cause similar effects in a distinct epidermal keratinocyte, primary normal human epidermal keratinocytes (NHEK), by using the same ELF-EMF exposure system and experimental design. In NHEK cells, ELF-EMFs exerted no effects on cell growth, cell proliferation, cell cycle distribution, and the activation of ATM signaling pathway. This study elucidated that the two types of epidermal keratinocytes differently responded to ELF-EMFs. To further validate this finding, the NHEK and HaCaT cells were simultaneously exposed to ELF-EMFs in the same incubator for 168 h and observed the cell growths. The results of simultaneous exposure in the two cell types showed that the NHEK and HaCaT cells exhibited distinct responses to ELF-EMFs. Thus, the biological effects of ELF-EMFs in epidermal keratinocytes are cell type specific. The findings may partially explain the inconsistent results of previous studies when comparing results across various experimental models. Based on the precise control of the ELF-EMF exposure and rigorous sham-exposure experiments in this study, the experiments at the transcriptional, protein and cellular levels all consistently supported the conclusion.