Prior to light exposure, the cells were gradually starved to synchronize the cell cycle. Both cell lines were first cultured with low-glucose DMEM (Euroclone S.p.A.) supplemented with 0.1% FBS and 1% penicillin/streptomycin for 24 h and then DMEM without phenol red (Lonza Group, Ltd.), also complemented with 0.1% FBS and 1% penicillin/streptomycin for a further 24 h.

Samples were exposed to sham, blue, or red single-color LEDs in a specific incubator at 37°C and 5% CO2 for 3 days. Constant darkness was considered the sham light source, while light exposures were performed in a 12-h light/dark cycle (12L:12D). High-power blue and red LEDs (LD W5AM and LH W5AM Golden DRAGON® Plus, respectively; Osram) were used as the light sources. The LED viewing angle was 170° and the cells were placed at 14 cm above the light sources. The homogenous distribution of light and the spectrum of emission of each monochromatic LED were previously verified using an illuminance meter (CL-70; Konica Minolta Sensing, Inc.). The dominant wavelength was 465 nm for blue and 658 nm for red LEDs. Irradiance at peak wavelength at the cell surface was 0.84 W/m2 for blue and 1.10 W/m2 for red LEDs, corresponding to the same total spectrum irradiance of 28.50 W/m2 for both light sources. The light energy transferred each day to cells was 1.23 J/mm2. The light exposure was set to reproduce the solar radiation as precisely as possible (63). Blue and red lights were specifically selected to test the opposite sides of the spectrum of visible light. Interference between light sources was prevented by using black curtains; sham exposure was additionally ensured by wrapping the plate with aluminum foil. LEDs in the incubator were fixed on an aluminum tank by thermal conductive paste, and the water circulation inside the tank (Amersham Multitemp III; GE Healthcare) extracted the heat generated by the LEDs, so they could work at a constant temperature. These conditions assured a constant electric current and therefore a constant emitted energy. Air circulation inside the incubator was ensured using a fan. To exclude any thermal effects, the temperature at the cell level was verified and constantly measured during experiments with Thermochron iButton DS1922L (Maxim Integrated). On the fourth day from the time of the start of sham or light exposure, the cells were transferred to the previous incubator and started the cisplatinum treatment.