Extracellular environment as one mediator of blue light-induced mitochondrial suppression

Objectives: The current study tested the hypothesis that the extracellular environment mediates mitochondrial suppression of oral epithelial cells and fibroblasts by blue light. Methods: We exposed Balb fibroblasts (Balb), normal human epidermal keratinocytes (NHEK), and oral squamous carcinoma cells (OSC2) to blue light (30-120 J/cm²) in different cell-culture media and in phosphate buffered saline (PBS). Mitochondrial activity (MTT method) was used to assess cellular response 72 h post-light exposure. Cell-culture media were replaced or supplemented before or after light exposure to assess the variables of exposure time and medium degradation as mediators of blue light-induced effects. Results: Mitochondrial activity of NHEK was not suppressed by exposure to blue light regardless of extracellular conditions. The mitochondrial activity of OSC2 and Balb cells was suppressed most when cells were exposed to light in cell-culture medium (versus PBS). Blue light suppressed mitochondrial activity more when irradiated medium remained in contact with the cells at least 1 h, indicating a time-dependence of the medium effects. Neither a replacement nor a supplementation of medium components reduced blue light-induced mitochondrial suppression. Significance: Our results suggest that tissue environments influence cellular responses to blue light and that these environments should be considered when assessing any biological effects of blue light during the photopolymerization of restorative resins.