Photoprotection of skin against ultraviolet A damage

Ultraviolet Radiation Exposure: Public Health Concerns

The North American public maintains an attitude that equates the acquisition of a suntan with health. However, prolonged exposure to ultraviolet radiation from the sun can lead to sunburn, premature skin aging, immunosuppression, and skin cancer. Misconceptions about the risks of tanning beds and the effectiveness of sunscreens are common. The public must be better informed about the importance of and the need for effective sun protection by means of clothing and hats, the proper use of sunscreen, and the avoidance of prolonged sun exposure during the time of maximal solar insolation.

Ultraviolet a Radiation and The Need for Protection

Erythema is a readily visible, often painful, natural phenomenon indicative of prolonged cutaneous exposure to ultraviolet (UV) radiation in the UVB range. As a result, early sunscreens were designed to provide adequate protection from UVB radiation. However, the continuous rise in the melanoma incidence rate alludes to the existence of an established photocarcinogen from which we are not well protected. Recently, scientists have been evaluating the photodamaging effects caused by wavelengths other than UVB. They have presented evidence that suggests that UVA exposure poses a greater hazard than previously acknowledged. For this reason, more emphasis must be placed on the development of broad-spectrum sunscreens providing adequate UVA and UVB protection. Additionally, regular sunscreen application must be advocated, proactive sun-protective practices must be implemented, and greater public awareness of the harmful effects of UVA and UVB radiation must be improved on, with the hope of decreasing the rate of cutaneous malignant melanoma.

Mexoryl: Broad-Spectrum Ultraviolet a Photoprotection

The deleterious effects of long-wave (320–400 nm) ultraviolet (UV) radiation on human skin have been recognized for decades. Human exposure to UV radiation may induce skin pigmentation and sunburn, cutaneous connective tissue alterations (photoaging), immunosuppression, and the development of skin cancers. Public awareness campaigns on the need for photoprotection advocate the regular use of sunscreens. Consumer demand and an expanding knowledge of the adverse effects of UV exposure have fueled the continual development of novel sunscreen formulations. Two organic UV filters, terephthlylidene dicamphor sulphonic acid (Mexoryl SX, L'Oréal, Paris, France) and drometrizole trisiloxane (Mexoryl XL, L'Oréal), provide effective protection from UV irradiation and offer improved safety profiles in terms of protection from UVA radiation. This article details the photoprotective benefits of Mexoryl SX and Mexoryl XL.

Kinetics and Mechanism of the Addition of Nucleophiles to α,β-Unsaturated Thiol Esters

Compounds containing the UV-absorbing chromophores p-methoxycinnamate, p-methoxycinnamide, or anthranilate and an alpha,beta- or alpha,beta,gamma,delta-unsaturated thiol ester (crotonyl or sorboyl) have been prepared. These compounds are subject to nucleophilic attack at the C=C conjugated to the thiol ester carbonyl group. The kinetics of the reactions of these thiol esters with N-acetyl-l-cysteine (NAC), N-acetylcysteamine, and N(2)-acetyl-L-lysine (NAL) have been studied, and the thiol addition products have been identified. The reaction rates increased at higher pH, and the reaction of NAC thiolate with a crotonyl thiol ester in 1:1 (v/v) acetonitrile/aqueous HEPES exhibited buffer catalysis as a result of protonation of the enolate intermediate. At the same concentration, NAC underwent approximately 300-fold more reaction than NAL with a crotonyl thiol ester at pH 9.8. Additionally, a crotonyl thiol ester was found to be 7.9 times more reactive than a sorboyl thiol ester toward NAC addition. These unsaturated thiol esters may serve as a means of covalently binding UVA and UVB sunscreens to the outer layer of skin to provide long-lasting protection.

Photostability of sunscreen products influences the efficiency of protection with regard to UV-induced genotoxic or photoageing-related endpoints

BACKGROUND

Acute as well as chronic sun exposure induces biologically damaging effects in skin including photoageing and cancer. Ultraviolet (UV)A radiation is involved in this process; it is therefore important that sunscreen products provide efficient and stable protection in this range of wavelengths.

OBJECTIVES

This study based on in vitro approaches was performed to demonstrate that photostability is an essential requirement to protect against UVA-induced genetic and dermal alterations.

METHODS

The protection afforded by two sunscreen products, differing with regard to their photostability, was studied using biological markers related to the genotoxic or photoageing impact of UVA or simulated solar UV radiation (UV-SSR). Comet assay was used to assess direct DNA breakage, photo-oxidized purines and lomefloxacin-induced DNA breaks in nuclei of normal human keratinocytes in culture. In similar conditions, detection of p53 accumulation was performed. The use of reconstructed skin in vitro allowed us to use a three-dimensional model to analyse the dermal and epidermal damage induced by UVA or UV-SSR exposure. Abnormal morphological features of the tissue as well as fibroblast alterations and matrix metalloproteinase-1 release induced by UV exposure have been studied after topical application of products on the skin surface.

RESULTS

The results showed that the photostable product afforded better protection with regard to all the criteria studied, compared with the photounstable product.

CONCLUSIONS

These data demonstrate that the loss of absorbing efficiency within the UVA wavelength domain due to photoinstability may have detrimental consequences on cell function and lead to impairments that have been implicated in genotoxic events as well as in the photoageing process.

The effects of derivatives of the nitroxide tempol on UVA-mediated in vitro lipid and protein oxidation

Derivatives of tetramethylpiperidines are extensively employed in polymers to prevent photooxidation, and their stabilizing effect is attributed to the activity of the nitroxide radical derived from the parent amine. In this study, we examined the photoprotective effect of a commercial polymer photostabilizer, HALS-1, its corresponding nitroxide, bis(2,2,6,6-tetramethyl-piperidine-1-oxyl-4-yl)sebacate (TINO), and two derivatives of the piperidine nitroxide TEMPOL, 2,2,6,6-tetramethyl-piperidin-4-acetyloxy-1-oxyl (TEMP2) and 2,2,6,6-tetramethyl-piperidin-4-octanoyloxy-1-oxyl (TEMP8) synthesized by us, in liposomes exposed to ultraviolet A (UVA) radiation. For comparison, the UVA-absorber, 4-tert-butyl-4'-methoxydibenzoylmethane (Parsol 1789) used in many suncream formulations, was also included. The nitroxide TINO resulted extremely efficient at inhibiting aldehydic breakdown products deriving from 30 min exposure of liposomes to UVA and the protection was dose-dependent (10-100 microM). The corresponding amine HALS-1 was the least efficient while protection increased in the order: TEMP2 < Parsol 1789 < TEMP 8. HALS-1, TINO, and the two TEMPOL derivatives were also tested in a simple protein system consisting of bovine serum albumin (BSA) exposed to UVA. In this case, these compounds did not inhibit nor enhance UVA-mediated protein carbonyl formation in BSA. The differences in protection between the compounds are discussed in relation to their chemical reactivity, UVA-absorbing capacities, and their molecular structure. Overall, the results obtained envisage the potential use of nitroxide compounds as topical antioxidants.

Oxidative stress-independent depletion of epidermal vitamin A by UVA

In hairless mice, epidermal vitamin A (retinol and retinyl esters) is strongly decreased following a single exposure to UVB. Here, using the same mouse model, we studied the effects of UVA on epidermal vitamin A content, lipid peroxidation, and CRBP-I expression, as well as the putative prevention of vitamin A depletion or lipid peroxidation by topical alpha-tocopherol. An acute exposure to UVA completely depleted epidermal vitamin A with EC50 of 0.25 and 0.5 J per cm2 for retinyl esters and retinol, respectively; these values were 0.1 J per cm2 for both retinoids under UVB exposure. CRBP-I expression was increased 2-fold 8 h following UVA exposure (10 J per cm2), and this increase persisted for at least 16 h. A single UVA exposure induced a concentration-dependent epidermal lipid peroxidation (EC50 = 3.5 J per cm2) giving rise to 55.4 +/- 4.2 nmol lipid peroxides per g at 20 J per cm2, whereas UVB, up to 1 J per cm2, did not increase the basal concentration of 6.7 +/- 0.9 nmol lipid peroxides per g. On the other hand, topical menadione induced a concentration-dependent lipid peroxidation, but did not affect vitamin A content. Pretreatment with alpha-tocopherol (i) did not inhibit UV-induced vitamin A depletion, (ii) completely inhibited the increased lipid peroxidation induced by UVA or menadione, and (iii) accelerated reconstitution of epidermal vitamin A after UVB but not UVA induced depletion. Thus acute UVA induced both epidermal vitamin A depletion and lipid peroxidation, UVB induced only vitamin A depletion, and menadione induced only a lipid peroxidation; topical alpha-tocopherol prevented lipid peroxidation but not vitamin A depletion. These observations indicate (i) that CRBP-I neither provides protection to UVB- and UVA-induced epidermal vitamin A depletion, nor interferes significantly with reconstitution, and (ii) that the UV-induced vitamin A depletion and lipid peroxidation in mouse epidermis are unrelated processes. UV light does not destroy epidermal vitamin A through an oxidative stress but probably by a photochemical reaction in which UV radiations at about 325 nm give the corresponding activation energy.