Bystander Effects in Osteoblasts and Osteoclasts: Comparison Between X- and Proton-Irradiation.

Purpose: This study investigated radiation-induced bystander responses (RIBR) in osteoblasts and osteoclasts and compared x-rays with protons regarding RIBR effects. Materials and Methods: MC3T3-E1 subclone 4 (preosteoblasts), differentiated MC3T3-E1 (osteoblasts), and differentiated RAW264.7 (osteoclasts) were irradiated with x-rays (225 kVp) and protons with a linear energy transfer of -2.3 keV/µm. A trans-well insert coculture system, allowing the transfer of soluble factors without contact between irradiated and nonirradiated cells, was used to study medium-mediated RIBR. The range of intended dose was 0.5 to 2 Gy for both x-ray and proton-irradiation. The RIBRs of nonirradiated cells were evaluated based on the expression of 53BP1 at 1 h postirradiation and micronuclei assay at 72 h or later. Results: RIBR was evident in bystander cells regardless of the irradiated/nonirradiated cell combination. The percentage of 53BP1-positive cells was 3.2% to 4.2% in controls and 8.6% to 11.7% in bystander cells (P < .05). Similarly, binucleated cells with micronuclei were 3.7% to 6.0% in controls versus 13.1% to 15.4% in bystanders (P < .05). No differences in RIBR were observed based on radiation type or dose. Conclusion: This study provides preliminary evidence that RIBR occurs in osteoblasts and osteoclasts, irrespective of cell combination, dose, or radiation type, suggesting that both x-rays and protons may induce comparable bystander effects in bone-related cells.