ReviewThe role of antioxidants in photoprotection: A critical review
Section snippets
Part I: Free radicals
A free radical is defined as a species that can exist independently with one or more unpaired electrons.1 In living systems, free radicals are predominantly represented as reactive oxygen species (ROS), taking form as oxygen-centered oxidizing agents. The most common oxygen-based ROS are: superoxide anion (O2−•), peroxide, hydroxyl radical (OH•), hydroxyl ion, and singlet oxygen (1O2), an excited state of molecular oxygen. ROS are volatile and unstable. In biological systems, ROS add electrons
Cellular damage from free radicals
Exposure to excess UV irradiation and pollutants leads to a pro-oxidant state. The resulting oxidative stress can impact the genetic integrity of a living organism. Whereas UVB directly damages DNA, UVA acts by ROS intermediates. ROS-induced DNA damages can lead to the formation of a modified guanine nucleotide (8-hydroxyguanine), single-stranded breaks, and oxidized pyrimidine bases.17, 18 These damages, although predominantly UVA related, have been observed in UVB-irradiated cells.19
Photoaging
Harman26 first proposed the free radical theory of aging in 1956 stating that free radical accumulation was contributing to the cumulative changes seen in aging. Indeed, free radical damage on the skin by chronic ROS and UV stress plays a major role in photoaging (Fig 4). After UV exposure, ROS trigger the release of proinflammatory cytokines and growth factors.8, 27 Specifically, factors activation protein-1 (AP-1) and nuclear factor-κB (NF-κB) up-regulate key matrix metalloproteinases (MMP)
Innate defense system against free radicals
Human skin has an elaborate enzymatic and nonenzymatic AOx defense network against ROS (Table I). The key AOx enzymes include SOD, catalase, and glutathione (GSH) peroxidase. SOD catalyzes the conversion of two volatile superoxide radicals into less volatile H2O2 and oxygen. H2O2 is further reduced to water and oxygen with the aid of catalase and GSH peroxidase (Fig 2). The nonenzymatic AOxs can occupy lipid- and water-soluble compartments of the cell, and the concentration and activity levels
Part II: Topical antioxidants
There is a growing trend in incorporating AOxs in sunscreens and skin care products to replenish the natural reservoirs in the skin. Topical AOxs have the potential to diminish the ROS generated from the UVA radiation. In the following section, common topical AOxs and their effectiveness as a component of photoprotection are reviewed, and additional compounds with AOx properties are featured in Table II.
Part III: Antioxidants in photoprotection
Sunscreen remains one of the most widely adopted strategies by the public to protect themselves from UVR. However, because of inadequate application and compensatory exposure where users of sunscreens tend to stay out in the sun longer, the degree of UV protection is much lower in practice than stated in the product labels. Furthermore, current sunscreens on the market tend to offer more UVB than UVA protection. Sunscreens may not offer adequate protection against UVA-induced ROS. In fact,
Conclusion
ROS from endogenous and exogenous sources, such as UVR and pollution, can damage the DNA, lipid membrane, and protein structures, and also play a role in the acceleration of photoaging and the development of skin cancer. Although the body’s innate AOx defense can neutralize ROS, these protective agents may be overwhelmed and depleted when faced with an excessive amount of oxidative stress. Delivery of topical AOxs has the potential to provide additional benefits, but there remain many
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Funding sources: None.
Disclosure: Dr Wang is a consultant for L’Oreal. Ms Chen and Dr Hu have no conflicts of interest to declare.