Found within various areas the human body, such as the skin, hair, pupils or irises of the eyes, as well as in certain areas of the brain, melanin is a complex pigment that is most notable for its responsibility in determining skin color.
As one of the most important natural protective agents within the skin, melanin exhibits important antioxidant and radical scavenging properties that protect epidermal cells from the harsh effects of ultraviolet (UV) radiation1.
To utilize photoprotective properties, researchers from the University of California (UC) San Diego have synthesized melanin-like polydopamine nanoparticles (MelNPs), and studied the way in which these MelNPs protect human keratinocytes from UV damage.
Originating from their hypothesis that synthetic MelNPs could mimic melanosomes, which are naturally occurring organelles that are responsible for the synthesis, storage and transport of melanin, and be taken up by keratinocytes to offer the same photoprotection as natural melanin provides in the skin.
To test this hypothesis, the UC San Diego team synthesized spherical MelNPs of a size distribution of approximately 200 nm, which was later confirmed by transmission electron microcopy (TEM), scanning electron microscopy (SEM) and dynamic light scattering (DLS)2.
Appearing black in color when in its monomer aqueous solution, the MelNPs exhibited a broad UV-vis absorption that ranged from 250 to 850 nm, which is consistent with the absorption of extracted eumelanin, a type of naturally occurring melanin.
To test whether the MelNPs resembled the behavior of naturally occurring melanin, a concentration of 0.02 MelNPs were incubated with human epidermal keratinocytes (HEKa) cells and observed at 4 h, 1 day, 2 days and 3 days after the in vitro treatment2. While previous concentrations of 0.4 and 0.1 mg/mL of MelNPs were studied, the researchers found that these higher concentrations caused the nanoparticles to adhere to the cell membrane.
One of the most predominantly deleterious effects of UV radiation on mammalian cells is its ability to cause DNA damage by inducing both mutagenic and cytotoxic DNA lesions. To test the ability of the MelNPs to prevent such irreversible damage, the researchers analyzed DNA damage in the HEKa cells by using a red fluorescent antibody against phosphorylated H2AX, which is a common byproduct of lethal genomic DNA damage.
While negative control HEKa cells exhibited a dramatic increase in their DNA damage and decrease in cell viability, HEKa cells that were incubated with MelNPs displayed a significantly smaller amount of DNA damage2.
In addition to this finding, the researchers found that there was no heat generation in the MelNPs following exposure to UV radiation, which therefore further supports the ability of the MelNPs to offer enhanced photoprotection.
The use of nanoparticles in sunscreen products is no new topic, as several manufacturers utilize either zinc oxide and/or titanium dioxide nanoparticles in their product formulations. Zinc oxide, for example, is a useful additive due to its consistent stability when exposed to sunlight, as well because of its ability to provide a greater protection from UV rays as compared to another approved sunscreen chemicals3.
While further research is needed to fully understand the extent of nanoparticles to reduce the risk of UV damage and not penetrate the epidermis to enter other tissues, the research conducted by the group of UC San Diego researchers highlights a natural sunscreen alternative that still manipulates its particle size for its advantage.
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Outside of its potential applications for use in skin protection products such as sunscreen, the MelNPs could inspire the development of novel therapies to mimic the biological functions of natural melanins.
Several melanin-defective related diseases, such as albinism and vitiligo, completely lack effective treatments, and due to this pigment insufficiency, patients with such diseases often are more susceptible to acquiring skin cancer.
The successful uptake and distribution of the MelNPs that was displayed in the UC San Diego studies confirmed the ability of these synthetic systems to act as artificial melanosomes.
- “What is Melanin?” – News-Medical Life Sciences
- “Mimicking Melanosomes: Polydopamine Nanoparticles as Artificial Microparasols” Y. Huang, Y. Li, et al. ACS Central Science. (2017). DOI: 10.1021/acsentsci.6b00230.
- “Nanoparticles in Sunscreens” – Environmental Working Group
- Image Credit: Shutterstock.com/solar22