UV irradiation‐induced zinc dissociation from commercial zinc oxide sunscreen and its action in human epidermal keratinocytes

Abstract

Zinc oxide (ZnO) is an active ingredient in sunscreen owing to its properties of broadly filtering the ultraviolet (UV) light spectrum and it is used to protect against the carcinogenic and photodamaging effects of solar radiation on the skin. This study investigated the dissociation of zinc (Zn(2+) ) from ZnO in commercial sunscreens under ultraviolet type B light (UVB) irradiation and assessed the cytotoxicity of Zn(2+) accumulation in human epidermal keratinocytes (HEK). Using Zn(2+) fluorescent microscopy, we observed a significant increase in Zn(2+) when ZnO sunscreens were irradiated by UVB light. The amount of Zn(2+) increase was dependent on both the irradiation intensity as well as on the ZnO concentration. A reduction in cell viability as a function of ZnO concentration was observed with cytotoxic assays. In a real-time cytotoxicity assay using propidium iodide, the treatment of UVB-irradiated ZnO sunscreen caused a late- or delayed-type cytotoxicity in HEK. The addition of a Zn(2+) chelator provided a protective effect against cellular death in all assays. Furthermore, Zn(2+) was found to induce the production of reactive oxygen species (ROS) in HEK. Our data suggest that UVB irradiation produces an increase in Zn(2+) dissociation in ZnO sunscreen and, consequently, the accumulation of free or labile Zn(2+) from sunscreen causes cytotoxicity and oxidative stress.

Emerging In Vitro Models for Safety Screening of High‐Volume Production Nanomaterials under Environmentally Relevant Exposure Conditions

“…Zinc oxide is another prime material candidate in the field of photoactive materials and is often discussed hand‐in‐hand with TiO 2 . Like TiO 2 , it falls into the category of semiconductor and has a broad spectrum adsorption with a bandgap of 3.2 eV,190 high refractive index of 1.95–2.10191 and the ability to absorb UVA and UVB 192. It has been studied heavily and even employed in numerous photoactive applications ranging from solar cells, photovoltaics to the most common UV‐filters in sunscreens.…”

“…Figure 201 reveals this mechanism in detail. ROS may accentuate ZnO's toxicity as shown by Chin et al202 Although ZnO‐induced ROS spikes have been well reported,192, 200, 201, 203–205 well planned dissolution studies must run parallel to cytotoxicity and ROS‐measurement studies in order to correctly attribute the source of ROS production to the nanoparticles or the ions.…”

“…Lisa Martorano et al192 proved UV irradiation‐induced zinc dissolution in commercial zinc oxide sunscreens. Photodegradation of zinc was mentioned earlier as a drawback of ZnO in the photocatalytic field, and this may well be a connected phenomenon whereby UV induces a greater availability of Zn 2+ ions which therefore cause higher toxicity.…”

Zinc oxide nanoparticles: a 90-day repeated-dose dermal toxicity study in rats

“…ZnO NPs have been reported to elicit various adverse cellular effects, including genotoxicity 2 and cytotoxicity. 3 It has also been suggested that ultraviolet B irradiation causes increased dissociation of Zn 2+ from ZnO and that the consequent accumulation of free or labile Zn 2+ from sunscreen results in cytotoxicity and oxidative stress. 3 However, the mechanisms underlying these adverse effects are not fully characterized.…”

“…3 It has also been suggested that ultraviolet B irradiation causes increased dissociation of Zn 2+ from ZnO and that the consequent accumulation of free or labile Zn 2+ from sunscreen results in cytotoxicity and oxidative stress. 3 However, the mechanisms underlying these adverse effects are not fully characterized. Decreased collagen content in the skin due to oxidative stress at the site of application of ZnO has been reported.…”