High-energy visible light at ambient doses and intensities induces oxidative stress of skin-Protective effects of the antioxidant and Nrf2 inducer Licochalcone A in vitro and in vivo

Abstract

Background: Solar radiation causes skin damage through the generation of reactive oxygen species (ROS). While UV filters effectively reduce UV-induced ROS, they cannot prevent VIS-induced (400-760 nm) oxidative stress. Therefore, potent antioxidants are needed as additives to sunscreen products.

Methods: We investigated VIS-induced ROS formation and the photoprotective effects of the Nrf2 inducer Licochalcone A (LicA).

Results: Visible spectrum of 400-500 nm dose-dependently induced ROS in cultured human fibroblasts at doses equivalent to 1 hour of sunshine on a sunny summer day (150 J/cm² ). A pretreatment for 24 hours with 1 µmol/L LicA reduced ROS formation to the level of unirradiated cells while UV filters alone were ineffective, even at SPF50+. In vivo, topical treatment with a LicA-containing SPF50 + formulation significantly prevented the depletion of intradermal carotenoids by VIS irradiation while SPF50 + control did not protect.

Conclusion: LicA may be a useful additive antioxidant for sunscreens.

Keywords: antioxidant; licochalcone A; reactive oxygen species; sunscreen; visible light.

Figure 1

Absorption spectra of the SPF 50 + sunscreen products with or without LicA used in this study

Figure 2

Reactive oxygen species formation in cultured human fibroblasts depending on wavelength. Cultured cells were exposed to 150 J/cm² of visible light >400 nm, >450 nm, >500 nm, and >585 nm, respectively. The Oriel 1600 W Solar Simulator filtered for VIS irradiation was used for the experiments. Results of unirradiated cells were set to 100%; n = 9; mean ± SD. Significant differences were marked (*P < .05, ***P < .001)

Figure 3

Formation of ROS in cultured human fibroblasts induced by solar simulated UV, VIS, and water‐filtered IRA. Cells were irradiated with various doses of UV and VIS and a fixed dose of water‐filtered IRA. Results of unirradiated cells were set to 100%; n = 7 for UV and VIS; n = 8 for wIRA; mean ± SD. Significant differences were marked (*P < .05, **P < .01, ***P < .001). An Oriel 1600 W Solar Simulator filtered for UV irradiation, another Oriel 1600 W Solar Simulator filtered for VIS irradiation, and a Hydrosun wIRA 505 for IRA irradiation were used

Figure 4

Reactive oxygen species formation in irradiated cultured human fibroblasts after pretreatment with LicA. After incubation with LicA at various concentrations, cells were exposed either to (A) UV (2.5 J/cm²) or (B) VIS >400 nm, 150 J/cm². An Oriel 1600 W Solar Simulator filtered for UV irradiation and another Oriel 1600 W Solar Simulator filtered for VIS irradiation were used. Gray bars show oxidative stress levels without irradiation, and black bars represent the additional oxidative stress induced by irradiation. Results of unirriadiated and untreated cells were set to 100%; n = 7; mean ± SD. Significant differences were marked (*P < .05, **P < .01, ***P < .001)

Figure 5

A, Setup for the irradiation of cultured cells under sunscreen protection. During irradiation, culture plates were covered by PMMA plates without1 or with2, 3 sunscreen SPF 50+/ UVA‐PF 40 containing LicA applied. To mimic penetration of antioxidant into the skin, some cultures were incubated with LicA prior to irradiation.3 B, ROS formation after VIS irradiation with SPF 50+/ UVA‐PF 40 sunscreen protection and with or without LicA. Fibroblast cultures were irradiated with 150 J/cm² VIS through a PMMA plate (white bar), a PMMA plate covered with sunscreen SPF 50+/UVA‐PF 40 (gray bar) or a PMMA plate with the same sunscreen applied and additionally 2 µmol/L LicA in the culture medium (black bar), respectively. Significant differences were marked (***P < .001). An Oriel 1600 W Solar Simulator filtered for VIS irradiation was used. C, Mean carotenoid levels after irradiation relative to initial values measured in vivo in the skin. Prior to irradiation with blue light (100 J/cm²), for the spectrum of the Skintrek^® PT3 filtered for UV irradiation see Figure S3 in Appendix S1, skin areas were either left untreated (white bar), or protected by a sunscreen (SPF 50+, UVA‐PF 40) containg only UV filters (gray bars) or the same sunscreen containing additionally LicA (black bar). Carotenoids in skin were measured in vivo by resonance Raman spectroscopy (n = 10; mean ± SD; *P < .05)