Interaction of TiO2 and ZnO NPs with human skin cells

Abstract

Nanotoxicology encompasses the study of the toxicity of nanoparticles (NPs) in order to better understand the adverse effects of NPs on human health and evaluate the health risks associated with the use of NPs. With the influx of large numbers of nanomaterials in many applications such as cosmetics, paints, and medicine, nanotechnology has been gaining immense attention in recent years. Titanium dioxide (TiO2) and Zinc oxide (ZnO) NPs are produced abundantly and are among the most ubiquitously used NPs in various consumer products, especially sunscreens. Penetration of these NPs through the intact skin barrier has been shown, albeit at low levels. Concerns over their increased penetration across the skin in cases of the compromised or altered skin barrier and the consequent physiological influences are real. However, understanding of nanoparticle-mediated toxicity is still limited. Therefore, the goal of this study is to assess the physiological effects of TiO2 and ZnO NPs interactions with primary human epidermal keratinocytes and dermal fibroblasts, with attention to accurate dosimetry measurements and particle solubility considerations. Since the current findings on NPs interaction with skin cells are mostly based on 2D monolayer cell culture and animal models, in this study, 3D human skin explants have also been used to better understand the effects of NPs in the skin tissue. Based on 2D monolayer studies, it was observed that both TiO2 and ZnO NPs were toxic to cells at higher concentration, inducing various cellular responses such as oxidative stress, apoptosis, inflammation, and autophagy. The cellular response was dependent on the dose and exposure period. TiO2 NPs induced lower cytotoxicity even at higher doses, whereas ZnO NPs caused acute cell death at a much lower dose. Comparing the cellular responses of two cell types, it was observed that both NPs were comparatively more toxic to keratinocytes than to fibroblasts. In keratinocytes, the onset of autophagy was observed at much lower/non-toxic doses. At higher/sub-toxic doses, both autophagy, and inflammation were induced as NPs mediated toxicity. This study, for the first time, shows that autophagy induction at higher doses of TiO2 NPs blocked p65 phosphorylation in keratinocytes, thereby blocking the inflammatory pathway. However, this phenomenon was not observed in keratinocytes exposed to ZnO NPs. In contrast to this, NPs induced autophagy was not observed in dermal fibroblasts. Furthermore, from the studies performed on human skin explants, it was observed that TiO2 and ZnO NPs did not cause any significant effects on skin physiology and wound healing. Although these NPs proved to be toxic on cells based on 2D monolayer culture, their significant effects on cells in 3D human skin explants were not observed. Both NPs were internalized by the keratinocytes at the wound edge, and notably, penetrated into the nuclei of the keratinocytes. The penetration and uptake of NPs were also dependent on the dose and exposure period. Although higher amounts of TiO2…