DNA repair is performed by enzymatic complexes that remove damage from DNA exposed to damaging agents such as Mitomycin C, UV radiation, Benzopyrene, Dimethyl-sulfate and the like. In DNA, UV primarily induces the formation of cyclo-butane pyrimidine dimers (CPD) and, to a lesser extent, of a variety of other chemical modifications. UV damage to DNA is generated immediately (within picoseconds) after absorption of the irradiated photon; repair takes generally 24-36 hours.

While analyzing the kinetics of removal of CPD from the DNA of cultured melanocytes exposed to UV, Sanjay Premi and his coworkers at Yale and in Japan, Brazil and France, observed that the accumulation of CPD did increase for a few hours after the end of the irradiation, thus indicating that “new” CPD were formed after the UV lamp was turned off. These new CPD were called “dark CPD” (dCPD) because they are formed in the absence of radiation, and were not generated in UV-exposed cells that do not contain melanin, such as fibroblasts or amelanotic melanocytes. Detailed mechanistical analysis suggested that a long-lived excited state of melanin, provoked by UV-induced peroxinitrates, is responsible for the formation of dCPD via energy transfer from excited melanin to cellular DNA.¹

Melanin’s Role in Skin

The relevance to human health of these findings was later pointed out by Antony Young’s team at the King’s College in London.² They exposed human volunteers of Fitzpatrick skin types I/II or VI to erythemally equivalent doses of UV from a solar simulator. At different times after exposure, biopsies were taken and CPDs were assessed by immunostaining as well as by HPLC-MS/MS. Computer imaging associated to immunostaining allowed them to analyze the accumulation of CPDs in three epidermal layers (basal, suprabasal-1 and suprabasal-2) whereas HPLC-MS/MS allowed them to perform the quantitative analysis of total epidermal CPD. It was observed that the level of CPD increased in all skin types after the end of the exposure to UV and peaked at about 1-2 hours post irradiation, thus confirming that dCPDs are formed in human skin after the end of the exposure to solar simulated radiation. Interestingly enough, neither CPD nor dCPD were found in the basal layer of the epidermis of Fitzpatrick type VI skins, where the concentration of melanin is highest and where the only melanin to be found is black eumelanin.

These results confirm the data reported by Young (summarized in reference 3) that in UV-exposed Black skin, the level of CPD in the basal layers is 60 times smaller than in Caucasian Fitzpatrick type I/II skin. Taken together these results provide a rationale for the understanding of the protection offered by eumelanin to Black skin against UV-induced cutaneous diseases and underscore the need of deeper analyses to understand the roles played by phaeomelanin(s) in the generation of DNA damage by ultraviolet radiation.

Relevance to Skin Care

The results summarized above cast a new light (no pun intended) on the generation of DNA damage in human skin. They also warrant reconsidering the strategies followed to design sunscreens and day creams, as well as the so-called after-sun soothing products. It appears necessary to formulate sunscreen and day creams able to offer protection not only to hinder immediate DNA damage, but also to avoid dark CPD. It appears also reasonable to prepare topical after-sun products to be applied as soon as sun exposure ends. These formulas must be soothing and hinder the energy transfer from excited melanin to DNA which occurs for a few hours after the end of exposure to sunlight.

All this can’t be achieved without ingredients able to hinder the formation of dCPD.

A very recent study by the team of Sanjay Premi4 shows that such ingredients do actually exist. In this study, melanotic and non-melanotic mouse melanocytes, neonatal human dermal fibroblasts and human HaCaT keratinocytes were exposed to UV from a solar simulator in the absence or in the presence of several plant-derived phenolic alkanones, none of which absorbs UV radiation. The study was aimed at deepening and extending preliminary results indicating that 3-(4-hydroxy, 3-methoxybenzyl)-pentane-2,4 dione (INCI name Acetyl Zingerone) had the capability of hindering melanin chemiexcitation and the formation of dCPD in melanocytes.⁵

The results in reference 4 do confirm that Acetyl Zingerone hinders the formation of dark CPD in melanotic melanocytes. They also point out than another alkenone, 3-(4-Methoxy-benzyl)-Pentane-2,4 dione (MBPD), is extremely effective in hindering the formation of dCPD in melanotic melanocytes. This is another success of research in photobiology that has been able to identify non-cytotoxic molecules able to hinder the formation of dark CPD within a few years after the first observation of their “existence.”

Hindering Formation

The amount of dark CPD that are formed after the turning off of the UV lamp is of the order of more than 50% of the total CPD generated during the irradiation. Therefore the discovery that MBPD and Acetyl Zingerone hinder the formation of dCPD is more than just an academic curiosity. In addition, and surprisingly enough, Premi and coworkers observed that exposure to UV of neonatal human dermal fibroblasts in the presence of MBDP results in a dramatic, dose dependent decrease of the level of “immediate” CPD; that is, the ones formed by direct absorption of a UV photon during irradiation.⁴

This effect of MDBP is not observed in HaCaT keratinocytes. Seemingly paradoxically, Acetyl Zingerone at high concentrations is able to reduce the number of immediate CPD in HaCaT keratinocytes but not in fibroblasts. As a consequence of these observations, these two molecules have been tested to study their effect on DNA repair and have been found to enhance the expression of some of the genes participating in the DNA repair process. The different results observed with HaCaT keratinocytes and neonatal dermal fibroblasts could be dependent on the different stage of differentiation of these two cell types. This is to say that the next step is eagerly awaited, to find molecules able to boost DNA repair either by photo-chemically rendering pristine DNA or by increasing the activity of enzymes belonging to the DNA repair complex.  

References

1. Premi, S et al (2015) Chemiexcitation of Melanin Derivatives Induces DNA Photoproducts Long after UV exposure. Science 347 : 842-847
2. Fajuyigbe D, Douki T, van Dijk A, Sarkany RP, Young AR (2021) Dark cyclobutene pyrimidine dimers are formed in the epidermis of Fitzpatrick skin types I/II and VI in vivo after exposure to solar-simulated radiation. Pigment Cell Melanoma Res. 34 : 575-584
3. Young AR (2020) Comparison of Skin Photoprotection by Pigmentation and Sunscreens. J. Cosm. Sci. 71 : 217-226
4. Srivastava J, Young MM, Yadav VK, Phadatare PR, Meyer TA, Chaudhuri RK, Premi S (2023) The role of Acetyl Zingerone and its derivatives in inhibiting UV-induced, Incident, and Delayed Cyclobutane Pyrimidine Dimers. Antioxidants, 12 : 278-295 https://doi.org/10.3390/antiox12020278
5. Chaudhuri R, Meyer T, Premi S, Brash D. (2019) Acetyl Zingerone: An efficacious multifunctional ingredient for continued protection against ongoing DNA damage in melanocytes after sun exposure ends. Int J Cosmet Sci 42 : 36-45

---

Paolo Giacomoni, PhD
Insight Analysis Consulting
[email protected]
516-769-6904
 
Paolo Giacomoni acts as an independent consultant to the skin care industry. He served as Executive Director of Research at Estée Lauder and was Head of the Department of Biology with L’Oréal. He has built a record of achievements through research on DNA damage and metabolic impairment induced by UV radiation as well as on the positive effects of vitamins and antioxidants. He has authored more than 100 peer-reviewed publications and has more than 20 patents. He is presently Head of R&D with L.RAPHAEL—The science of beauty—Geneva, Switzerland.