An Observed Effect of Ultraviolet Radiation Emitted from Beta-Irradiated HaCaT Cells upon Non-Beta-Irradiated Bystander Cells

Previous research has shown that beta radiation can induce ultraviolet (UV) photon emission in human keratinocyte cells. Spectral analysis using a filter-based method in the ultraviolet range demonstrated that the strongest externally measureable photon emission was induced by beta radiation in the UVA range. In the current study, the potential biological implications of this UV photon emission from beta-irradiated cells were investigated. HaCaT human keratinocyte cells were irradiated with tritium ((3)H) and the photon emission induced was concurrently measured at the strongest externally measurable wavelength, 340 ± 5 nm, using a combination filter-photomultiplier tube system. Unirradiated reporter HaCaT cell cultures were also placed directly above (3)H-irradiated cells so that they would receive the induced secondary photons emitted from beta-irradiated cells, and the clonogenic survival in reporter cells was then assessed. Maximum photon emission (1207.04 ± 107.65 counts per second) was observed during irradiation of 2,000 cells/cm(2) with (3)H and the maximum reporter cell death (23.2 ± 0.9% reduction in survival) was observed under the same conditions. The measured photon emission from beta-irradiated cells and reporter cell death were strongly correlated (r = 0.977, P < 0.01). Placement of a polyethylene terephthalate filter, designed to eliminate >90% of UV wavelengths below 390 nm, between the directly irradiated and reporter cell layers was effective in nearly abolishing both 340 nm photon detection and reporter cell death in treated groups. Concurrent treatment of reporter cells with lomefloxacin during exposure to the secondary photons resulted in significantly increased cell killing, indicating a potential synergistic effect, while melanin treatment resulted in decreased reporter cell killing regardless of irradiation. These results suggest that secondary photons in the UV spectral range induced by beta irradiation play a role in inducing a response in neighboring non-beta-irradiated reporter cells.