Ornithine decarboxylase induction in psoralen-treated mouse epidermis used as a test of UV-A sunscreen potency

A novel in vivo model for evaluating agents that protect against ultraviolet A-induced photoaging

Increasing evidence demonstrates that ultraviolet A radiation (UVA) contributes to photoaging, which results in the accumulation of massive amounts of abnormal elastic material in the dermis of photoaged skin. To study UVA-induced photoaging in an in vivo system, we utilized a line of transgenic mice containing the human elastin promoter linked to a chloramphenicol acetyl transferase reporter gene. Our prior work demonstrates promoter activation in response to ultraviolet B radiation (UVB), UVA, and psoralen plus ultraviolet A radiation in the skin of these mice. The addition of psoralen (8-MOP) prior to administration of UVA results in substantial increases in promoter activation, as compared with UVA alone. To demonstrate the utility of these mice as a model of UVA-induced photodamage, we administered four lotions to the skin of our transgenic mice that included: a sunscreen containing octyl methoxycinnamate and benzophenone-3 with a sun protection factor (SPF) of 15, the UVA filter butyl methoxydibenzoylmethane, the SPF 15 sunscreen and the UVA filter together, and the lotion vehicle alone. Following sunscreen administration, mice received a single psoralen plus ultraviolet A radiation treatment. All sunscreens decreased chloramphenicol acetyl transferase activity with the SPF 15 sunscreen, the UVA filter, and the combination SPF 15 sunscreen and UVA filter, resulting in increasing degrees of protection against psoralen plus ultraviolet A radiation. These results demonstrate that this model functions as a rapid and sensitive model of UVA photodamage for the identification and comparison of compounds that protect against UVA-induced photoaging.

Hydrolysis of RRR-alpha-tocopheryl acetate (vitamin E acetate) in the skin and its UV protecting activity (an in vivo study with the rat)

Vitamin E acetate is often used rather than vitamin E as an ingredient of skin care products and dermatological preparations, because it lacks the free phenolic OH group. However, because of this the acetate as such is biologically inactive. In spite of this intrinsic inactivity, the skin is protected against the harmful effects of sunlight after topical application of vitamin E acetate. Therefore it is supposed that hydrolysis takes place in the skin and that the reaction product, the radical scavenger vitamin E, is responsible for the protection observed. In this in vivo study with the rat, we have investigated the hydrolysis of RRR-alpha-tocopheryl acetate (vitamin E acetate) in the epidermis in relation to UV radiation protection. (As a measure of protection, we used the UV-induced binding of 8-methoxypsoralen to epidermal biomacromolecules.) After a period of 5 h from a single application of vitamin E acetate, hydrolysis into free vitamin E was not observed. No protection was found at this time point, corresponding with the absence of vitamin E. After treatment for 5 days, consisting of one topical application daily, the percentage of acetate present in the stratum corneum which was hydrolysed into free vitamin E was less than 1%, whereas the corresponding value for the viable layer of the epidermis was about 5%. The hydrolysis of vitamin E acetate in the epidermis proceeded very slowly. As a result, the absolute amount of free vitamin E, found in the total epidermis after treatment for 5 days with the acetate, was only a few times higher than the normal level. Yet, this very small amount of free vitamin E proved to be sufficient for maximal protection in this animal model. The results show that vitamin E acetate acts as a prodrug, which very slowly releases minute amounts of active vitamin E.

UV-radiation protecting efficacy of thiols, studied with UVA-induced binding of 8-MOP and CPZ to rat epidermal biomacromolecules in vivo

The following topically-applied thiols were investigated with regard to their possible UV-radiation protective properties: captopril, cysteamine, ergothioneine, mesna, mercaptopropionylglycine, N-acetyl-cysteine and penicillamine. As a measure for protection the inhibition of in vivo irreversible photobinding of the labelled phototoxic drugs chlorpromazine (CPZ) and 8-methoxypsoralen (8-MOP) to rat epidermal biomacromolecules was used. Ergothioneine, mesna and penicillamine did not show any effect; probably, as a result of their charge they are not able to enter the stratum corneum. Captopril, cysteamine, mercaptopropionylglycine and N-acetylcysteine showed a considerable inhibition of CPZ and 8-MOP photobinding. Captopril and N-acetylcysteine were clearly the most potent whereas cysteamine was the least effective. Captopril, mercaptopropionylglycine and N-acetylcysteine appeared to have a wider action range and to be a more effective protector than dl-alpha-tocopherol and di-butyl-hydroxytoluene. Cysteamine and mercaptopropionylglycine were only capable of protecting the stratum corneum. Captopril and N-acetylcysteine on the other hand showed an additional dose-dependent inhibition of photobinding to the viable epidermis. Gradually with increasing time after application, the protecting efficacy with regard to the viable layer of the epidermis decreased; the duration of protection depending on the dose.

Thiols as potential UV radiation protectors: an in vitro study

The following thiols were investigated with regard to their possible UV-radiation protective properties: captopril, cysteamine, ergothioneine, mesna, mercaptopropionylglycine, N-acetylcysteine, and penicillamine. As a measure for protection, the inhibition of in vitro irreversible photobinding of the labeled phototoxic drugs chlorpromazine (CPZ) and 8-methoxypsoralen (8-MOP) to protein and DNA was used. Besides photobinding to biomacromolecules, the photodegradation of CPZ and the formation of promazine (PZH) and hydroxypromazine (PZOH) were measured as well. Because of the H-atom and electron donating capacity of the thiols, the ratio [PZOH]/[PZH] was expected to be decreased and the photodegradation of CPZ was expected to be higher in the presence of thiols. Maximum inhibition of CPZ photobinding ranged for the different thiols between 21-100% (DNA) and 17-87% (human serum albumin). All thiols enhanced the photodegradation of CPZ (19-84%) and inhibited the ratio [PZOH]/[PZH] (90-97%). 8-MOP photobinding to human serum albumin was also clearly inhibited (75-96%), but remarkably less to DNA (2-41%). This study indicates that thiols are able to cope with a variety of reactive species. Scavenging of radicals, quenching of singlet molecular oxygen species and reaction with excited states seem to be essential mechanisms involved with this process.

Evaluating the UVA photoprotection of sunscreens with murine skin edema

The acute and chronic deleterious effects of UVA on skin have prompted a growing interest in developing effective UVA-photoprotective sunscreens. The quantification of their UVA photoprotection remains, however, a major problem. In the present study, murine skin edema induced by 8-methoxypsoralen plus UVA (PUVA) is evaluated as a screening method for quantifying the UVA photoprotection of commercially available sunscreens. The PUVA-induced murine skin edema is provoked on the dorsa of female hairless albino mice and measured with a hand-held micrometer. A clear time course and a well-defined dose-response relationship are demonstrated. Therefore, a UVA-photoprotection factor (UVA-PF) could be defined by dividing the minimal edema dose with sunscreen by the minimal edema dose without sunscreen. The UVA-PF values obtained with this method were quantitatively and qualitatively very similar to those obtained in 8-methoxypsoralen-photosensitized murine skin by using the number of sunburn cells as the biologic end point and were qualitatively similar to UVA-PF values obtained in human skin using phototoxic erythema and UVASUN-induced tanning as the parameter. It is concluded that PUVA-induced murine skin edema offers an objective, reproducible, and easily applicable screening method for quantifying the degree of UVA photoprotection of a sunscreen.

Evaluating the UVA protection of sunscreens

Because the protection factor of sunscreens concerns only UVB protection, usually nothing is known about the protection offered in the UVA range. Using different methods, we compared six commercially available sunscreens to determine the UVA protection factor and, thus, to select the most appropriate sunscreen. Two clinical methods on human skin (inhibition of UVA-induced tanning with the use of a high-intensity UVA source and inhibition of methoxsalen plus UVA-induced phototoxicity) were compared with a method in animals (inhibition of UVA-induced sunburn cell production in mice treated with methoxsalen) and with two in vitro techniques (solution-dilution and sandwich spectrophotometry). We conclude that all five methods used give a quantitative estimate of UVA protection, but none can be accepted as a standard because the UVA protection factor varies according to the method used and the reading time.

Modification of sunburn cell production in 8-MOP sensitized mouse epidermis: a method of assessing UVA sunscreen efficacy

The UVA(320-400 nm) photoprotection of four commercially available sunscreens with different sun protection factors was evaluated in 8-methoxypsoralen (8-MOP) treated mouse epidermis with the number of sunburn cells (SBCs) as the end-point. SBC production in 8-MOP sensitized mouse epidermis with and without application of sunscreens is dose-related. The difference between the slopes of the dose-response curves of the control group and the four sunscreens was highly significant (p less than 0.001). It appears that the SBC production was modified both by the dose of the UVA irradiation administered and by the UVA photoprotection of the four sunscreens. It could be concluded that the number of SBCs in 8-MOP sensitized mouse epidermis is a useful parameter to quantify the degree of UVA photoprotection of sunscreens.