In a recent study published in Scientific Reports, a group of researchers evaluated how melanin content and distribution affect ultraviolet (UV)-induced deoxyribonucleic acid (DNA) damage in skin, using reconstructed human epidermis (RHE) models.
Study: Significance of melanin distribution in the epidermis for the protective effect against UV light. Image Credit: rangizzz/Shutterstock.com
Background
While ultraviolet radiation (UVR) is crucial for vitamin D and endorphins, its overexposure increases the risk of skin cancer through DNA damage.
This damage occurs via UV-B absorption and UV-A-induced radicals, leading to specific DNA lesions. Thanks to melanin, skin pigmentation offers substantial photoprotection, notably reducing cancer rates in darker skin.
Melanin's effectiveness is attributed to its UV-blocking, antioxidant, and radical-neutralizing properties. Nonetheless, the role of melanin is complex, as it may also enhance cell sensitivity to UVR damage.
This contradiction prompts a need for further research to understand melanin's dual effects on photoprotection and photosensitization in skin cancer dynamics.
About the study
In the present study, RHE models developed from primary epidermal keratinocytes and melanocytes of Asian-Caucasian and Afro-American donors were categorized into tanned and light based on their melanin content, established through a newly validated method on ex vivo human skin.
This method involved melanin extraction and spectrometric analysis at 500 nm wavelength, correlating melanin levels with the Individual Typological Angle (ITA°), a measure of skin pigmentation.
Ex vivo human skin samples, sourced from healthy individuals of various ethnic backgrounds undergoing surgery, were used to calibrate the melanin quantification process. These samples allowed researchers to classify the RHE models according to melanin content, facilitating a comparison between light and tanned models.
The study assessed DNA damage in these models following UV irradiation, employing immunohistochemical staining to quantify damage.
Additionally, the effect of UV exposure on radical formation was examined using Electron Paramagnetic Resonance (EPR) spectroscopy, revealing insights into the oxidative stress induced by UV light in different melanin concentrations.
A critical aspect of the research was investigating melanin distribution within the epidermis, utilizing techniques like Fontana-Masson staining and Two-Photon Excited Fluorescence Lifetime Imaging Microscopy (TPE-FLIM).
These methods provided a detailed view of melanin's localization, contributing to understanding its protective versus potential photosensitizing effects.
Study results
After extracting melanin from human epidermis samples and RHE, researchers quantified the total melanin content spectrometrically by measuring absorbance at 500 nm.
In ex vivo skin samples, melanin content varied significantly, correlating strongly with the skin's ITA°, indicating a methodological validation. This approach was then applied to RHE, revealing distinct melanin levels between tanned and light models.
The correlation between melanin content and ITA° was used to categorize RHE models by skin color, confirming the method's applicability across different epidermal sources.
The study further evaluated DNA damage through immunohistochemical staining, quantifying the extent of damage by the presence of cyclobutane-pyrimidine dimers (CPD) in cells post-UVR exposure.
Results showed significant DNA damage across all RHE models immediately after exposure, with variances in damage levels based on the type of UVR and the model's pigmentation.
Notably, tanned RHE models exhibited more damage than light ones, especially after specific types of UV irradiation. This damage assessment highlighted potential photoprotection discrepancies tied to melanin content and distribution.
Additionally, the research investigated radical formation post-irradiation, finding that tanned RHE models produced more free radicals than light models under certain conditions, suggesting melanin's complex role in the skin's response to UV exposure.
Contrary to in vivo skin, where melanin typically encapsulates keratinocyte nuclei, providing a protective barrier, tanned RHE models displayed a non-homogeneous melanin distribution. This misallocation potentially undermines melanin's protective efficacy against UVR.
The examination extended to the melanin coverage within basal cells, revealing a stark contrast between in vivo conditions and tanned RHE models.
In vivo, melanin distribution was relatively uniform across different skin types, whereas tanned RHE showed minimal melanin presence, diverging significantly from expected patterns. This inconsistency underscores a fundamental difference in melanin's protective mechanism in vitro versus in vivo.
Conclusions
To summarize, melanin plays a dual role in the human body, acting as a skin pigment that protects against solar radiation while also exhibiting photosensitizing properties linked to skin pathologies like melanoma.
This complex molecule's protective and harmful effects on the skin are still not fully understood.
Research using reconstructed RHE of tanned and light skin types has sought to delve deeper into melanin's multifaceted roles, examining how melanin content and distribution influence UV-induced DNA damage and the generation of reactive oxygen species (ROS).
These studies have shown that despite higher melanin levels in tanned models suggesting greater protection, all models exhibited significant DNA damage following UV exposure, challenging the notion of melanin's protective efficacy.
The distribution of melanin, particularly its concentration in certain cells and absence in others, may contribute to its photosensitizing effects, leading to increased free radical production and DNA damage.
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