Effects of Titanium Dioxide Nanoparticles on UV-Exposed Human Dermal Fibroblasts

Titanium dioxide (TiO₂) nanoparticles (NPs) are commonly found as an active ingredient in mineral sunscreens due to their strong absorbance of UV-B irradiation. Although the absorption spectrum underpinning therapeutic applications of TiO₂ is well characterized, recent studies suggesting potential genotoxicity of TiO₂ NPs in vitro¹ have called into question the efficacy and safety of titanium nanoparticle' cosmetic use.<br/>Human dermal fibroblasts (HDFs), the cell type analyzed in this study, are integral to synthesizing collagen and thus, maintaining extracellular matrix (ECM) structures throughout the skin. Age of HDFs is correlated with diminished collagen production and, by extension, weakened ECM quality and integrity. Likewise, deleterious outcomes such as reduced actin filament integrity and morphological changes resulting in reduced collagen synthesis or destruction via apoptosis² have been observed in HDFs as a result of ultraviolet (UV) radiation exposure. This research aims to clarify the effectiveness of TiO₂ NPs in reducing UV-induced damage to HDFs by assessing the impact of varying rutile TiO₂ NP (50 nm) concentrations on the morphology and survival of fibroblast cells exposed to UV dosages of intensity 7 J/cm³.<br/>HDFs collected from patients aged 19, 29, 62, and 71 with passage numbers ranging from 7 to 8 were cultured, treated with TiO₂ NPs or left undisturbed, and subjected to none, single, or double exposure to UV light. In single-exposure trials, HDFs were treated at 0.4 mg/ml TiO₂ concentration, and in double-exposure trials, HDFs were treated at 0.2 and 0.4 mg/ml TiO₂ concentrations. Cell counts were conducted to quantify cell viability post-UV exposure for all exposed samples, elucidating the effect of TiO₂ NPs with respect to UV-induced cellular damage.<br/>Decreased cell viability was observed after TiO₂ addition alone in all trials. Younger HDFs exhibited greater survival after TiO₂/1x UV treatment versus UV exposure alone; survival of older UV-exposed HDFs decreased under the same criterion. Intensified cell death was observed after 2x UV exposure. Variable change in survival was observed in 0.2 mg/ml TiO₂/2x UV groups compared with 2x UV alone (decline in ages 19, 62, increase in ages 29, 71); survival consistently declined in 0.4 mg/ml TiO2/2x UV groups. Exemplifying such trends were HDFs aged 19, in which control, 0.2 mg/ml TiO2, and 0.4 mg/ml TiO₂ groups showed death rates of 32.9%, 36.6%, and 43.7%, respectively, and HDFs aged 62, in which otherwise identical groups experienced death rates of 30.5%, 50.0%, and 62.6%, respectively.<br/>Upon immunofluorescence staining and imaging, UV-exposed fibroblasts exhibited modified, less elongated morphology, potentially signifying damage at the cellular level. For HDFs aged 29 and 71, aspect ratio decreased by 30.4% and 58.2% after 1x UV treatment and 47.9% and 61.0% after 2x UV treatment, respectively. Absorption of TiO2 NPs in significant quantities occurred uniformly. Though altered shape was again observed with increasing NP concentration, TiO₂ was shown to strongly modulate such structural changes, with decreases in aspect ratio after 2x UV as low as 15.6% and 6.1% for 0.2 mg/ml TiO₂/2x UV groups in HDFs 29 and 71, respectively.<br/>The study examined differential fibroblast response to TiO2 and/or UV exposure in evaluating the potential of TiO₂ NPs to modulate cellular response to UV irradiation. Findings corroborate TiO₂ NPs' ability to induce damage in HDFs, highlighting a need for greater investigation of health risks inherent to titanium dioxide's cosmetic applications. Though physical sunscreens maintain health and environmental advantages over chemical alternatives, potential adverse effects must be understood to provide transparency and guarantee buyer safety.<br/>Work supported by the Louis Morin Charitable Trust.<br/><br/>¹Cao, Y., et al. <i>Toxics</i>, 11(11), 882 (2023).<br/>²Rehemtulla, A., et al. <i>The Journal Of Biological Chemistry</i>, 41(272), 25783-25786 (1997).