Stage‐specific phenotypic and transcriptional alterations in <scp>HaCaT</scp> keratinocytes exposed to acute and chronic blue light

Despite evidence that visible light (VL) has similar effects on human skin as those of UVA, VL is often viewed as harmless. High SPF sunscreen prevents erythema but can lead to overexposure to UVA and VL, with unknown consequences. To explore the impact of chronic blue light exposure, we irradiated (50 J/cm², λ = 408 nm, three times a week) human immortalized keratinocytes under acute (3 irradiations), intermediate (14 irradiations), and chronic (42 irradiations) blue light exposure, monitoring phenotypic and gene expression changes. Chronically exposed keratinocytes exhibit increased nuclei area, chromatin alterations, higher proliferation, and apoptosis resistance, mirroring the consequences of chronic UVA exposure. While acute exposure upregulated keratinization and downregulated tissue repair and apoptosis genes, chronically exposed cells had upregulated genes involved in energy metabolism and oxidative phosphorylation, and downregulated genes were enriched for immune and inflammatory responses. Specific transcription factors were identified in both the acute and chronic stages, some of which have been associated with UVB exposure. IRF1, EGR1, ELF3, FOSL1, and CENPX, SRF, CEBPB, KLF4 were identified in the acute and chronic stages, respectively. We identified some changes in chronically irradiated keratinocytes similar to malignant transformation, emphasizing the need for further research on the long-term impacts of blue light exposure on human skin.