Anti-senescence and anti-apoptotic effects of immature Citrus unshiu peel ethanol extract on ultraviolet B-irradiated skin keratinocytes.

Immature Citrus unshiu peel has garnered attention due to its potent antioxidant effects. In this study, we aimed to investigate the protective effects of immature citrus peel extract (ICPEE) against ultraviolet B (UVB) irradiation-induced senescence and apoptosis in human keratinocytes (HaCaT). Cells were pretreated with ICPEE for 1 h and exposed to UVB. For cell senescence and apoptosis measurements, UVB was applied three times at 10 mJ/cm² every 6 h and once at 30 mJ/cm², respectively. ICPEE alleviated reduced cell viability and increased reactive oxygen species (ROS) levels induced by UVB irradiation. UVB-induced increases in intracellular calcium levels, senescence-associated β-gal activity, and G₁ phase arrest were also restored by ICPEE. Furthermore, ICPEE reverted the UVB-induced changes in the expression of cyclin D1, CDK4, cyclin E, CDK2, matrix metalloproteinases (MMP-1, MMP-2, and MMP-9), tissue inhibitor of metalloproteinases-1, and apoptosis-related proteins. The caspase inhibitor Z-VAD-FMK significantly restored UVB-impaired cell viability, comparable to ICPEE, suggesting the anti-apoptotic activities of ICPEE. Additionally, cell viability and apoptotic body measurement using SP600125, a JNK inhibitor, showed that ICPEE reduced UVB-induced expression of JNK and upstream factor SEK protein. ICPEE exhibited inherent UVB absorption capability. The findings of this study suggest that ICPEE is an effective natural material for protecting against skin aging owing to its strong photoaging and apoptosis inhibitory properties.