Keratinocytes exposed to ultraviolet A and ultraviolet B radiation induce responses in melanocytes

Ultraviolet (UV) radiation exerts a strong influence on skin homeostasis and directly affects communication between keratinocytes and melanocytes. This study investigated how exposure of keratinocytes to UVA and UVB radiation alters cellular signaling with melanocytes using conditioned medium, that is, culture medium collected from keratinocytes after irradiation, to evaluate its effects on mitochondrial activity, proliferation, viability, and cell cycle progression of melanocytes. Keratinocytes were irradiated with UVA (4 J.cm^-2) or UVB (20 mJ.cm^-2), and the conditioned medium was collected 24 h after irradiation. Subsequently, irradiated or non-irradiated melanocytes were treated with the medium. The results showed that the conditioned medium from keratinocytes, especially those exposed to UVA radiation, significantly increased mitochondrial activity and melanocyte proliferation. Although direct UVA exposure reduced melanocyte viability, treatment with conditioned medium promoted cellular recovery over time. The conditioned medium obtained from UVB-irradiated keratinocytes also stimulated proliferation, although to a lesser extent than the medium from non-irradiated keratinocytes. Additionally, it was observed that the conditioned medium from UVA-irradiated keratinocytes induced cell cycle progression, increasing the proportion of melanocytes in the S and G2/M phases. These findings indicate that soluble factors secreted by irradiated keratinocytes play an important role in modulating the functional responses of melanocytes, potentially contributing to adaptive mechanisms related to UV radiation exposure.