Extracellular Environment Modulates Blue light-induced Tumor Cell Damage

Objectives: We have reported that blue light (380-500 nm, 5-60 J/cm²) suppresses growth of tumor cells but not normal cells. Here, we tested the hypothesis that extracellular environments mediate these effects. Photoactive substances such as riboflavin generate reactive oxygen species (ROS) that may contribute to blue light effects. If true, blue light treatment of tumors could be tissue-specific and modulated by modifying the extracellular environment. Methods: Balb fibroblasts, normal human epidermal keratinocytes (NHEK), or oral squamous carcinoma cells (OSC2) were exposed (n = 6) to blue light (30-120 J/cm²) in media or phosphate buffered saline (PBS). Cell-culture media also were replaced or supplemented before or after irradiation to assess the effect of time or medium conditions on blue light effects (ANOVA, Tukey). In other experiments, riboflavin or H₂O₂ were added to medium to test synergism of blue light and extracellular sources of ROS. Results: NHEK were not metabolically suppressed by blue light exposure under any experimental condition (p > 0.05, succinate dehydrogenase (SDH) activity, MTT). SDH activity of OSC2 and Balb cells was suppressed 60-90% when cells were exposed to light in cell-culture medium vs. 0-20% in PBS (p < 0.05), suggesting that photoexposure altered a medium component that contributed to SDH suppression. H₂O₂ or riboflavin supplementation of medium enhanced suppression of tumor cell metabolism at all light doses (p < 0.01). Replacement of medium immediately post-irradiation negated light-induced suppression of tumor SDH activity, but this reversal was not observed if medium was replaced > 1 h post-irradiation, suggesting that medium effects were time-dependent. Conclusions: Our previous studies implicated intracellular ROS as mediators of blue light effects, but our current results suggest that extracellular ROS also contribute to these effects. Thus, the potential therapeutic benefits of blue light for cancer therapy may depend on specific tissue environments during light exposure.