Induction of oral cancer apoptosis signaling by blue light

OBJECTIVES: Our lab previously has shown that specific doses of blue light (400-500 nm) induce apoptosis of tumor cells in culture but are not toxic to normal cells. This study examined the effect of blue light treatment on important stress responsive signaling pathways in normal and tumor cells. METHODS: Normal human epidermal keratinocytes (NHEK) and oral squamous carcinoma cells (OSC2) were seeded onto coverslips in six well plates, and treated with blue light (45 J/cm²) delivered from a quartz-tungsten-halogen dental curing light (VIP, Bisco, 600 mW/cm² output). At 30 min, 2 h, and 6 h post-light treatment, cells were fixed and stained with antibodies specific for the oxidative stress-responsive protein transcription factors, NFκB and Nrf2, as well as the Nrf2-regulated stress protein, heme oxygenase 1 (HO-1). Fluorescein isothiocyanate (FITC) – conjugated secondary antibodies were used to identify antigen-antibody complexes. Intracellular distribution and fluorescent intensity were visualized through a Nikon 600 microscope and quantified using MetaMorph imaging software. Conditioned media collected prior to cell fixation was used for ELISA quantification of the NFκB-regulated pro-apoptotic cytokine, tumor necrosis factor alpha (TNFα). RESULTS: We found that NFκB and Nrf2 both were found in the nucleus of OSC2 cells following blue light exposure indicating enhanced activation and were predominantly cytosolic (excluded from the nucleus) by 6 h post-light treatment. OSC2 levels of intracellular HO-1 and secreted TNFα were increased at 6 h post-light treatment. NHEK responded to light treatment by activating Nrf2 and enhancing HO-1 production but showed minimal NFκB response and TNFα secretion. CONCLUSIONS: These data show that OSC2 tumor cell signaling in response to blue light exposure differs from that of normal cells, resulting in enhanced activation of NFκB and possible TNFα-induced apoptosis.