Effects of wavelength, fluence, irradiance, and irradiation mode of visible light on melanogenesis in B16F10 melanoma cells.

High-fluence visible light (VL) has been reported to induce cutaneous pigmentation similarly to ultraviolet radiation. However, VL is a polychromatic light composed of different wavelengths. More experimental studies are required to elucidate the effects of VL parameters on melanin synthesis. In this study, using murine B16F10 melanoma cells as an in vitro model, we systematically investigated how wavelength, fluence, irradiance and irradiation mode jointly determine the melanogenic response to VL. We revealed that 448 nm blue light stimulated melanogenesis, whereas 560-733 nm light globally suppressed it, with 595 nm yellow light exhibiting the strongest inhibition. Furthermore, a biphasic response was observed: both 560 nm and 595 nm yellow light inhibited melanogenesis at 3-12 J/cm2 but markedly enhanced it at 48 J/cm2. 595 nm light suppressed melanin synthesis at 5-20 mW/cm2, yet switched to stimulation at ≥ 40 mW/cm2. Additionally, our results suggested that the 595 nm pulsed PBM (3000 Hz, 40 %-80 % duty cycle) outperformed continuous PBM, whereas the 20 % duty cycle (with a peak irradiance of 25 mW/cm2) abolished the inhibitory effect, indicating that high irradiance activates pro-melanogenic defense pathways. Experimental data indicated that these differences arise primarily from the distinct effects of light parameters on tyrosinase activity and the mRNA levels of microphthalmia-associated transcription factor (MITF), TYR, TRP1, and TRP2. In conclusion, the regulatory effect of VL on melanogenesis is dependent on the selection of these parameters. These data provide an experimental basis for optimizing LED phototherapy parameters and support subsequent in-depth mechanistic studies in human cutaneous melanocytes. In the future, optimized VL parameters may balance the efficacy and safety of phototherapy in skin disorders caused by pigmentation dysregulation, such as vitiligo and melasma.