Role of sunscreen formulation and photostability to protect the biomechanical barrier function of skin

The impact of sunscreen formulations on the barrier properties of human skin are often overlooked leading to formulations with components whose effects on barrier mechanical integrity are poorly understood. The aim of this study is to demonstrate the relevance of carrier selection and sunscreen photostability when designing sunscreen formulations to protect the biomechanical barrier properties of human stratum corneum (SC) from solar ultraviolet (UV) damage. Biomechanical properties of SC samples were assayed after accelerated UVB damage through measurements of the SC's mechanical stress profile and corneocyte cohesion. A narrowband UVB (305–315 nm) lamp was used to expose SC samples to 5, 30, 125, and 265 J cm^?2 in order to magnify damage to the mechanical properties of the tissue and characterize the UV degradation dose response such that effects from smaller UV dosages can be extrapolated. Stresses in the SC decreased when treated with sunscreen components, highlighting their effect on the skin prior to UV exposure. Stresses increased with UVB exposure and in specimens treated with different sunscreens stresses varied dramatically at high UVB dosages. Specimens treated with sunscreen components without UVB exposure exhibited altered corneocyte cohesion. Both sunscreens studied prevented alteration of corneocyte cohesion by low UVB dosages, but differences in protection were observed at higher UVB dosages indicating UV degradation of one sunscreen. These results indicate the protection of individual sunscreen components vary over a range of UVB dosages, and components can even cause alteration of the biomechanical barrier properties of human SC before UV exposure. Therefore, detailed characterization of sunscreen formulation components is required to design robust protection from UV damage.     

The effects of manganese doping on UVA absorption and free radical generation of micronised titanium dioxide and its consequences for the photostability of UVA absorbing organic sunscreen components.

The effect of manganese doping on the free radical generation rate, free radical scavenging and UVA absorption properties of micronised sunscreen grade titania has been studied with respect to enhancement of the UVA photostability of test sunscreen formulations containing the organic UVA absorber Parsol 1789. Manganese doping has been shown to increase the UVA:UVB absorption ratio of titania, reduce free radical generation rates by over 90%, and provide free radical scavenging behaviour. Adding manganese-doped titania to a test formulation incorporating Parsol 1789 shows that manganese doping increases UVA attenuation stability by up to 3 times the amount achieved by comparable commercial undoped titania materials. HPLC data shows this to be related to an improved stabilisation of the organic sunscreen components. Manganese doped titania shows improved efficacy over undoped titania in sunscreen formulations containing organic UV absorbers.     

Effects of ultraviolet radiation on human cutaneous nerve fibres

Due to an increasing number of skin diseases as a result of exposure to ultraviolet (UV) radiation, it is necessary to evaluate the effectiveness of new skin care formulations with broad-spectrum sunscreens.
Objectives:  This study aims to assess the status of nerve fibres in healthy human skin, to quantify effects of UV radiation on nerve endings, and to evaluate neuroprotective effects of new skin care formulations against UV exposure damage.
Methods:  Samples were obtained from 34 female patients enrolled for plastic surgery and were immediately treated (10 min) with three emulsions: Cream 1, Cream 2 (placebo) and a sunscreen with sun protection factor 15 (SPF15). Control samples and those treated with the cream emulsions were exposed to UVA and UVB for 60 min. Nerve fibres were identified by immunofluorescence using a monoclonal antibody (anti-human CD56/NCAM). Cell damage was assessed by image analysis.
Results:  Several cellular nervous structures were identified in the skin samples, including free nerve endings. UVA and UVB significantly decreased (40–60%) density of nerve endings in the control samples and those treated with placebo (Cream 2) or SPF15 (all P  < 0.001). Cream 1 completely blocked effects of UV radiation on nerve endings ( P  > 0.05 vs. control).
Conclusions:  Quantification of cell damage induced by UV radiation provides useful information for identification of new skin care compounds with neuroprotective properties.     

In vitro photostability and photoprotection studies of a novel 'multi-active' UV-absorber

This paper reports on the synthesis and properties of a new UV-absorber (OC-NO) based on the most popular UV filter worldwide, ethylhexyl methoxycinnamate (OMC) in which the methoxy group has been replaced with a pyrrolidine nitroxide bearing antioxidant activity. This sunscreen active has therefore both UV-absorbing and antioxidant properties which could ideally address both the UV-B and UV-A skin photo-damage. For broad-spectrum coverage, the combinations of OC-NO with two commonly used UV-A absorbers (BMDBM and DHHB) were also studied. The results obtained reveal that OC-NO: (a) is as photostable as OMC after UV-A exposure; (b) acts as free radical scavenger as demonstrated by EPR and chemical studies; (c) reduces UV-A and UV-A+BMDBM induced lipid peroxidation in liposomes and cells, measured as reduced TBARS levels and increased C11-BODIPY red fluorescence, respectively; (d) has comparable antioxidant activity to that of vitamin E and BHT commonly used in skin care formulations; (e) is non-cytotoxic to human skin fibroblasts as assessed with the MTT assay when exposed to increasing doses of UV-A; and (f) OC-NO+DHHB is a promising, photostable broad spectrum UV-filter combination that concomitantly reduces UV-induced free radical damage. These results suggest that nitroxide/antioxidant-based UV-absorbers may pave the way for the utilization of 'multi-active' ingredients in sunscreens thereby reducing the number of ingredients in these formulations.     

Measuring sunscreen protection against solar-simulated radiation-induced structural radical damage to skin using ESR/spin trapping: development of an ex vivo test method

The in vitro star system used for sunscreen UVA-testing is not an absolute measure of skin protection being a ratio of the total integrated UVA/UVB absorption. The in vivo persistent-pigment-darkening method requires human volunteers. We investigated the use of the ESR-detectable DMPO protein radical-adduct in solar-simulator-irradiated skin substitutes for sunscreen testing. Sunscreens SPF rated 20+ with UVA protection, reduced this adduct by 40-65% when applied at 2 mg/cm(2). SPF 15 Organic UVA-UVB (BMDBM-OMC) and TiO(2)-UVB filters and a novel UVA-TiO(2) filter reduced it by 21, 31 and 70% respectively. Conventional broad-spectrum sunscreens do not fully protect against protein radical-damage in skin due to possible visible-light contributions to damage or UVA-filter degradation. Anisotropic spectra of DMPO-trapped oxygen-centred radicals, proposed intermediates of lipid-oxidation, were detected in irradiated sunscreen and DMPO. Sunscreen protection might be improved by the consideration of visible-light protection and the design of filters to minimise radical leakage and lipid-oxidation.     

UVA filters in sun-protection products: regulatory and biological aspects

This review of published in vitro and in vivo studies concerning the biological effects of ultraviolet A (UVA; 320-400 nm) radiation illustrates the evidence for combining UVA and UVB filters in sun-protection products. These data have led to the development of new sunscreens as well as methods to evaluate their efficacy. After listing the UVA filters available and briefly noting the requirements for a high SPF, broad-spectrum sunscreen, the methods for evaluating the level of UVA protection will be described. This article also summarizes several studies looking at the prevention of erythema, pigmentation, DNA damage, photoimmunosuppression, photoaging and photodermatoses. These data demonstrate in vitro and in vivo that only well-balanced UVA-UVB sunscreens, absorbing over the entire UV spectrum are able to prevent or significantly reduce the associated biological damage.     

Changes of superoxide dismutase, catalase and glutathione peroxidase in the corneal epithelium after UVB rays. Histochemical and biochemical study

In this study, the effects of UVA and UVB rays on antioxidant enzymes (superoxide dismutase, glutathione peroxidase, catalase) were examined in the corneal epithelium. The corneas of albino rabbits were irradiated with a UV lamp generating UVA (365 nm wavelength) or UVB rays (312 nm wavelength), 1 x daily for 5 min, from a distance of 0.03 m, over 4 days (shorter procedure) or 8 days (longer procedure). In contrast to UVA rays, which did not evoke significant disturbances, UVB rays changed the activities of antioxidant enzymes. The longer repeated irradiation with UVB rays was performed, the deeper the observed decrease in antioxidant enzymes. The shorter procedure evoked a more profound decrease of glutathione peroxidase and catalase (the enzymes cleaving hydrogen peroxide) than of superoxide dismutase, an enzyme scavenging superoxide radical and producing hydrogen peroxide during the dismutation reaction of a superoxide free radical. This may contribute to an insufficient hydrogen peroxide cleavage at the corneal surface and danger to the cornea from oxidative damage. After the longer procedure (UVB rays), the activities of all antioxidant enzymes were very low or completely absent. In conclusion, repeated irradiation of the cornea with UVB rays evokes a deficiency in antioxidant enzymes in the corneal epithelium, which very probably contributes to the damage of the cornea (and possibly also deeper parts of the eye) from UVB rays and the reactive oxygen products generated by them.     

Inorganic and organic UV filters: Their role and efficacy in sunscreens and suncare products

Minerals such as titanium dioxide, TiO₂, and zinc oxide, ZnO, are well known active semiconductor photocatalysts used extensively in heterogeneous photocatalysis to destroy environmental pollutants that are organic in nature. They are also extensively used in sunscreen lotions as active broadband sunscreens that screen both UVB (290-320 nm) and UVA (320-400 nm) sunlight radiation and as high SPF makers. When so photoactivated by UV light, however, these two particular metal oxides are known to generate highly oxidizing radicals (OH and ) and other reactive oxygen species (ROS) such as H₂O₂ and singlet oxygen, ¹O₂, which are known to be cytotoxic and/or genotoxic. Hydroxyl (OH) radicals photogenerated from photoactive TiO₂ specimens extracted from commercial sunscreen lotions [R. Dunford, A. Salinaro, L. Cai, N. Serpone, S. Horikoshi, H. Hidaka, J. Knowland, FEBS Lett. 418 (1997) 87] induce damage to DNA plasmids in vitro and to whole human skin cells in cultures. Accordingly, the titanium dioxide particle surface was modified to produce TiO₂ specimens of considerably reduced photoactivity. Deactivation of TiO₂ diminishes considerably, in some cases completely suppresses damage caused to DNA plasmids, to human cells, and to yeast cells compared to non-modified specimens exposed to UVB/UVA simulated solar radiation. The photostabilities of sunscreen organic active agents in neat polar and apolar solvents and in actual commercial formulations have been examined [N. Serpone, A. Salinaro, A.V. Emeline, S. Horikoshi, H. Hidaka, J. Zhao, Photochem. Photobiol. Sci. 1 (2002) 970]. With rare exceptions, the active ingredients undergo photochemical changes (in some cases form free radicals) and the sunscreen lotions lose considerable Sun protection efficacy only after a relatively short time when exposed to simulated sunlight UVB/UVA radiation, confirming the recent findings by Sayre et al. [R.M. Sayre, J.C. Dowdy, A.J. Gerwig, W.J. Shields, R.V. Lloyd, Photochem. Photobiol. 81 (2005) 452].     

Effect of inhibitors of oxygen radical and nitric oxide formation on UV radiation-induced erythema,immunosuppression and carcinogenesis

Abstract

We investigated whether supplementation of a sunscreen containing the UVB absorber 2-ethyl-hexyl-methoxycinnamate (cinnamate) with oxygen radical inhibitors (ORI) would improve protection from sunburn, immunosuppression and carcinogenesis. Mice were exposed to solar-simulated UV radiation (ssUV) containing a mixture of UVB and UVA. In initial studies, the ORI 2,2′-dipyridyl and N^G-monomethyl-L-arginine acetate (L-NMMA) were shown to prevent UVA-induced suppression of contact sensitivity (CS) in mice. Addition of these inhibitors to the sunscreen did not affect the sun protection factor (SPF), but lowered the level of edema when mice were exposed to ssUV. Combination of both inhibitors with the sunscreen, however, increased the SPF from 5 to 5.5. The immune protection factor (IPF) of the sunscreen was only 1.18, but addition of neither dipyridyl nor L-NMMA singly or in combination measurably improved immune protection. However, the ORI improved the ability of the sunscreen to prevent carcinogenesis. The results indicate that reactive oxygen or nitrogen species produced in response to UV radiation are important for erythema, immunosuppression and carcinogenesis, and addition of inhibitors improves the protective capacity of sunscreens.     

Effect of inhibitors of oxygen radical and nitric oxide formation on UV radiation-induced erythema, immunosuppression and carcinogenesis

We investigated whether supplementation of a sunscreen containing the UVB absorber 2-ethyl-hexyl-methoxycinnamate (cinnamate) with oxygen radical inhibitors (ORI) would improve protection from sunburn, immunosuppression and carcinogenesis. Mice were exposed to solar-simulated UV radiation (ssUV) containing a mixture of UVB and UVA. In initial studies, the ORI 2,2'-dipyridyl and N(G)-monomethyl-L-arginine acetate (L-NMMA) were shown to prevent UVA-induced suppression of contact sensitivity (CS) in mice. Addition of these inhibitors to the sunscreen did not affect the sun protection factor (SPF), but lowered the level of edema when mice were exposed to ssUV. Combination of both inhibitors with the sunscreen, however, increased the SPF from 5 to 5.5. The immune protection factor (IPF) of the sunscreen was only 1.18, but addition of neither dipyridyl nor L-NMMA singly or in combination measurably improved immune protection. However, the ORI improved the ability of the sunscreen to prevent carcinogenesis. The results indicate that reactive oxygen or nitrogen species produced in response to UV radiation are important for erythema, immunosuppression and carcinogenesis, and addition of inhibitors improves the protective capacity of sunscreens.     

Increased oxidative modification of albumin when illuminated in vitro in the presence of a common sunscreen ingredient: protection by nitroxide radicals

We previously reported on the ability of dibenzoylmethane (DBM) and a relative, Parsol 1789, used as a ultraviolet A (UVA)-absorbing sunscreen, to generate free radicals upon illumination, and as a consequence, to inflict strand breaks in plasmid DNA in vitro. This study has now been extended to determine the effects of Parsol 1789 and DBM on proteins, under UVA illumination, with the sole purpose of gaining more knowledge on the photobiological effects of sunscreen chemicals. Parsol 1789 (100 microM) caused a 2-fold increase in protein carbonyl formation (an index of oxidative damage) in bovine serum albumin (BSA) when exposed to illumination, and this damage was both concentration- and time-dependent. The degree of protein damage was markedly reduced by the presence of free radical scavengers, namely piperidinic and indolinonic nitroxide radicals, in accordance with our previous study. Vitamin E had no effect under the conditions used. The results obtained corroborate the fact that Parsol 1789 generates free radicals upon illumination and that these are, most probably, responsible for the protein damage observed under the conditions used in our system. However, at present, we cannot extrapolate from these results the relevance to human use of sunscreens; therefore, further studies should be necessary to determine the efficacy at the molecular and cellular level of this UVA-absorber in order to ascertain protection against photocarcinogenic risk.     

The suppression of immunity by ultraviolet radiation: UVA, nitric oxide and DNA damage.

We have examined the mechanism by which solar-simulated ultraviolet radiation (ssUV) suppresses memory immunity to nickel in allergic humans. In initial studies, we used inbred mice to determine the contribution of different wavebands to sunlight-induced immunosuppression. We found that low dose UVA can enhance memory, medium dose UVA (half the amount in one minimum erythemal dose of ssUV) is immunosuppressive, but higher doses protect from UVB. This is genetically dependent, as it is not observed in all mouse strains. UVA caused a similar dose-related change in recall immunity in humans. ssUV dose responses determined the limits of protection provided by sunscreens from immunosuppression in humans. Immune protection factors calculated from these data correlated with UVA protection, but not with sun protection factor, showing that in commercial sunscreens that provide good UVB protection, UVA protection limits prevention of immunosuppression. N(G)-monomethyl-l-arginine acetate (l-NMMA) was used to inhibit nitric oxide (NO) production and T4N5 liposomes containing T4 endonuclease V to enhance DNA repair. Sub-erythemal ssUV caused a dose-related local suppression of recall immunity to nickel in humans. l-NMMA and the liposomes protected the nickel reaction, suggesting that NO and DNA damage are mediators of UV-induced immunosuppression in humans.     

Illumination of human keratinocytes in the presence of the sunscreen ingredient Padimate-O and through an SPF-15 sunscreen reduces direct photodamage to DNA but increases strand breaks.

On illumination with simulated sunlight, the UVB-absorbing sunscreen chemical 2-ethylhexyl-4-dimethylaminobenzoate (Padimate-O) generates excited species which inflict non-ligatable strand breaks on DNA in vitro and it also becomes mutagenic to yeast in vivo. Padimate-O is known to penetrate human skin but its effects on human cells are not clear. Here, we first simulate the sunlight which penetrates human skin and use it to illuminate human keratinocytes. The DNA damage observed in terms of UV-endonuclease-sensitive sites (ESS) and direct strand breaks per kilobase (kb) of DNA per joule per square metre agrees well with that predicted from action spectra based on monochromatic light. Using plasmid DNA in vitro, we find a very similar pattern of results. Next, we simulate the spectrum that results when the incident light is first attenuated by a film of sunscreen (SPF-15; 2 mg/cm(2)) containing benzophenone-3 (a UVA absorber), octyl methoxycinnamate (a UVB absorber), and Padimate-O. If the sunscreen is not in contact with keratinocytes it reduces direct DNA damage from sunlight (ESS). However, any Padimate-O in contact with the cells substantially increases indirect damage (strand breaks) even though the film of sunscreen reduces direct photodamage. We estimate that applying an SPF-15 sunscreen which contains Padimate-O to human skin followed by exposure to only 5 minimum erythemal doses (MED) of sunlight could, while suppressing the formation of ESS, increase strand breaks in cells under the epidermis by at least 75-fold compared to exposure to 1 MED in the absence of sunscreen.     

Effect of UVA fluence rate on indicators of oxidative stress in human dermal fibroblasts

During the course of a day human skin is exposed to solar UV radiation that fluctuates in fluence rate within the UVA (290-315 nm) and UVB (315-400 nm) spectrum. Variables affecting the fluence rate reaching skin cells include differences in UVA and UVB penetrating ability, presence or absence of sunscreens, atmospheric conditions, and season and geographical location where the exposure occurs. Our study determined the effect of UVA fluence rate in solar-simulated (SSR) and tanning-bed radiation (TBR) on four indicators of oxidative stress---protein oxidation, glutathione, heme oxygenase-1, and reactive oxygen species--in human dermal fibroblasts after receiving equivalent UVA and UVB doses. Our results show that the higher UVA fluence rate in TBR increases the level of all four indicators of oxidative stress. In sequential exposures when cells are exposed first to SSR, the lower UVA fluence rate in SSR induces a protective response that protects against oxidative stress following a second exposure to a higher UVA fluence rate. Our studies underscore the important role of UVA fluence rate in determining how human skin cells respond to a given dose of radiation containing both UVA and UVB radiation.     

Synthesis,antioxidant and photoprotection activities of hybrid derivatives useful to prevent skin cancer

Chronic ultraviolet (UV) radiation exposure is a major cause of skin cancer. A novel series of hybrid derivatives (I–VIII) for use in sunscreen formulations were synthesized by molecular hybridization of t-resveratrol, avobenzone, and octyl methoxycinnamate, and were characterized. The antioxidant activity values for VIII were comparable than to those of t-resveratrol. Compounds I–III and VI demonstrated Sun Protector Factor superior to that of t-resveratrol. Compounds I and IV–VIII were identified as new, broad-spectrum UVA filters while II–III were UVB filters. In conclusion, novel hybrid derivatives with antioxidant effects have emerged as novel photoprotective agents for the prevention of skin cancer.     

Human erythema and matrix metalloproteinase-1 mRNA induction, in vivo, share an action spectrum which suggests common chromophores

Matrix metalloproteinase 1 (MMP-1) is widely regarded as a biomarker of photoageing. We tested the hypothesis that MMP-1 mRNA expression and erythema share a common action spectrum by comparing the effects of erythemally equivalent doses of UVB, UVA1 and solar simulated radiation (SSR) on acute MMP-1 mRNA expression in whole human skin in vivo. Our results show comparable MMP-1 expression with all three spectra, which supports our hypothesis. The sharing of an action spectrum implies common chromophores, one of which is likely to be DNA. We have previously shown that all spectra that we used readily induce cyclobutane thymine dimers (T<>T) in human epidermis in vivo but we lack quantitative data on damage to dermal DNA. This is important because we do not know if dermal MMP-1 induction occurs via direct damage to the dermis, or indirectly via damage to the epidermis. Our results show that UVB induces about 3 times more T<>T compared with erythemally equivalent doses of UVA1, which is similar to our published epidermal data. This supports previously published work that also implicates an unknown UVA1 chromophore for erythema and MMP-1 induction. However, the distribution of the dermal DNA damage varies considerably with spectrum. In the case of UVB it is primarily in the upper dermis, but with UVA1 it is evenly distributed. Thus, irrespective of chromophores, MMP-1 induction by direct dermal damage by both spectra is possible. The practical conclusions of our data are that the small (<5%) UVB content of solar UVR is likely to be the main cause of photoageing, at least in terms of MMP-1 expression. Furthermore, prevention of erythema by sunscreen use is likely to result in reduced MMP-1 expression.     

Protein, lipid, and DNA radicals to measure skin UVA damage and modulation by melanin

Afro-Caribbeans have a lower incidence of skin cancer than Caucasians, but the effectiveness of melanin as a photoprotective pigment is debated. We investigated the UVA and solar irradiation of ex vivo human skin and DMPO using electron spin resonance spectroscopy, to determine whether pigmented skin is protected by melanin against free radical damage. Initial ascorbate radicals in Caucasian skin were superseded by lipid and/or protein radical adducts with isotropic (a(H)=1.8 mT) and anisotropic spectra comparable to spectra in irradiated pig fat (a(H)=1.9 mT) and BSA. DNA carbon-centered radical adducts (a(H)=2.3 mT) and a broad singlet were detected in genomic DNA/melanin but were not distinguishable in irradiated Caucasian skin. Protein and lipid radicals (n=6 in Caucasian skin) were minimal in Afro-Caribbean skin (n=4) and intermediate skin pigmentations were variable (n=3). In irradiated Afro-Caribbean skin a shoulder to the melanin radical (also in UVA-irradiated pigmented melanoma cells and genomic DNA/melanin and intrinsic to pheomelanin) was detected. In this sample group, protein (but not lipid) radical adducts decreased directly with pigmentation. ESR/spin trapping methodology has potential for screening skin susceptibility to aging and cancer-related radical damage and for measuring protection afforded by melanin, sunscreens, and antiaging creams.     

Pigmentation effects of solar-simulated radiation as compared with UVA and UVB radiation

Different wavelengths of ultraviolet (UV) radiation elicit different responses in the skin. UVA induces immediate tanning and persistent pigment darkening through oxidation of pre-existing melanin or melanogenic precursors, while UVB induces delayed tanning which takes several days or longer to develop and requires activation of melanocytes. We compared the effects of a 2-week repetitive exposure of human skin to solar-simulated radiation (SSR), UVA or UVB at doses eliciting comparable levels of visible tanning and measured levels of melanins and melanin-related metabolites. Levels of eumelanin and pheomelanin were significantly higher in the order of SSR, UVB, UVA or unexposed control skin. Levels of free 5-S-cysteinyldopa (5SCD) were elevated about 4-fold in SSR- or UVB-exposed skin compared with UVA-exposed or control skin. Levels of protein-bound form of 5SCD tended to be higher in SSR- or UVB-exposed skin than in UVA-exposed or control skin. Total levels of 5-hydroxy-6-methoxyindole-2-carboxylic acid (5H6MI2C) and 6H5MI2C were higher in SSR- than in UVB-exposed or control skin. These results show that SSR is more effective in promoting delayed tanning than UVB radiation alone, suggesting a synergistic effect of UVA radiation. Furthermore, free 5SCD may serve as a good marker of the effect of SSR and UVB.     

3-hydroxypyridine chromophores are endogenous sensitizers of photooxidative stress in human skin cells

Photocarcinogenesis and photoaging are established consequences of chronic exposure of human skin to solar irradiation. Accumulating evidence supports a causative involvement of UVA irradiation in skin photo-damage. UVA photodamage has been attributed to photosensitization by endogenous skin chromophores leading to the formation of reactive oxygen species and organic free radicals as key mediators of cellular photooxidative stress. In this study, 3-hydroxypyridine derivatives contained in human skin have been identified as a novel class of potential endogenous photosensitizers. A structure-activity relationship study of skin cell photosensitization by endogenous pyridinium derivatives (pyridinoline, desmosine, pyridoxine, pyridoxamine, pyridoxal, pyridoxal-5'-phosphate) and various synthetic hydroxypyridine isomers identified 3-hydroxypyridine and N-alkyl-3-hydroxypyridinium cation as minimum phototoxic chromophores sufficient to effect skin cell sensitization toward UVB and UVA, respectively. Photosensitization of cultured human skin keratinocytes (HaCaT) and fibroblasts (CF3) by endogenous and synthetic 3-hydroxypyridine derivatives led to a dose-dependent inhibition of proliferation, cell cycle arrest in G2/M, and induction of apoptosis, all of which were reversible by thiol antioxidant intervention. Enhancement of UVA-induced intracellular peroxide formation and p38 mitogen-activated protein kinase-dependent stress signaling suggest a photooxidative mechanism of skin cell photosensitization by 3-hydroxypyridine derivatives. 3-hydroxypyridine derivatives were potent photosensitizers of macromolecular damage, effecting protein (RNase A) photocross-linking and peptide (melittin) photooxidation with incorporation of molecular oxygen. Based on these results, we conclude that 3-hydroxypyridine derivatives comprising a wide range of skin biomolecules, such as enzymatic collagen cross-links, B6 vitamers, and probably advanced glycation end products in chronologically aged skin constitute a novel class of UVA photosensitizers, capable of skin photooxidative damage.     

The deceptive nature of UVA tanning versus the modest protective effects of UVB tanning on human skin

The relationship between human skin pigmentation and protection from ultraviolet (UV) radiation is an important element underlying differences in skin carcinogenesis rates. The association between UV damage and the risk of skin cancer is clear, yet a strategic balance in exposure to UV needs to be met. Dark skin is protected from UV-induced DNA damage significantly more than light skin owing to the constitutively higher pigmentation, but an as yet unresolved and important question is what photoprotective benefit, if any, is afforded by facultative pigmentation (i.e. a tan induced by UV exposure). To address that and to compare the effects of various wavelengths of UV, we repetitively exposed human skin to suberythemal doses of UVA and/or UVB over 2 weeks after which a challenge dose of UVA and UVB was given. Although visual skin pigmentation (tanning) elicited by different UV exposure protocols was similar, the melanin content and UV-protective effects against DNA damage in UVB-tanned skin (but not in UVA-tanned skin) were significantly higher. UVA-induced tans seem to result from the photooxidation of existing melanin and its precursors with some redistribution of pigment granules, while UVB stimulates melanocytes to up-regulate melanin synthesis and increases pigmentation coverage, effects that are synergistically stimulated in UVA and UVB-exposed skin. Thus, UVA tanning contributes essentially no photoprotection, although all types of UV-induced tanning result in DNA and cellular damage, which can eventually lead to photocarcinogenesis.