The sunburn cell revisited: an update on mechanistic aspects

ATG5-mediated keratinocyte ferroptosis promotes M1 polarization of macrophages to aggravate UVB-induced skin inflammation

Autophagy participates in the regulation of ferroptosis. Among numerous autophagy-related genes (ATGs), ATG5 plays a pivotal role in ferroptosis. However, how ATG5-mediated ferroptosis functions in UVB-induced skin inflammation is still unclear. In this study, we unveil that the core ferroptosis inhibitor GPX4 is significantly decreased in human skin tissue exposed to sunlight. We report that ATG5 deletion in mouse keratinocytes strongly protects against UVB-induced keratinocyte ferroptosis and skin inflammation. Mechanistically, ATG5 promotes the autophagy-dependent degradation of GPX4 in UVB-exposed keratinocytes, which leads to UVB-induced keratinocyte ferroptosis. Furthermore, we find that IFN-γ secreted by ferroptotic keratinocytes facilitates the M1 polarization of macrophages, which results in the exacerbation of UVB-induced skin inflammation. Together, our data indicate that ATG5 exacerbates UVB-induced keratinocyte ferroptosis in the epidermis, which subsequently gives rise to the secretion of IFN-γ and M1 polarization. Our study provides novel evidence that targeting ATG5 may serve as a potential therapeutic strategy for the amelioration of UVB-caused skin damage.

Monoterpenoid aryl hydrocarbon receptor allosteric antagonists protect against ultraviolet skin damage in female mice

The human aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is a pivotal regulator of human physiology and pathophysiology. Allosteric inhibition of AhR was previously thought to be untenable. Here, we identify carvones as noncompetitive, insurmountable antagonists of AhR and characterize the structural and functional consequences of their binding. Carvones do not displace radiolabeled ligands from binding to AhR but instead bind allosterically within the bHLH/PAS-A region of AhR. Carvones do not influence the translocation of ligand-activated AhR into the nucleus but inhibit the heterodimerization of AhR with its canonical partner ARNT and subsequent binding of AhR to the promoter of CYP1A1. As a proof of concept, we demonstrate physiologically relevant Ahr-antagonism by carvones in vivo in female mice. These substances establish the molecular basis for selective targeting of AhR regardless of the type of ligand(s) present and provide opportunities for the treatment of disease processes modified by AhR.

The CaSR Modulator NPS-2143 Reduced UV-Induced DNA Damage in Skh:hr1 Hairless Mice but Minimally Inhibited Skin Tumours

The calcium-sensing receptor (CaSR) is an important regulator of epidermal function. We previously reported that knockdown of the CaSR or treatment with its negative allosteric modulator, NPS-2143, significantly reduced UV-induced DNA damage, a key factor in skin cancer development. We subsequently wanted to test whether topical NPS-2143 could also reduce UV-DNA damage, immune suppression, or skin tumour development in mice. In this study, topical application of NPS-2143 (228 or 2280 pmol/cm²) to Skh:hr1 female mice reduced UV-induced cyclobutane pyrimidine dimers (CPD) (p < 0.05) and oxidative DNA damage (8-OHdG) (p < 0.05) to a similar extent as the known photoprotective agent 1,25(OH)₂ vitamin D3 (calcitriol, 1,25D). Topical NPS-2143 failed to rescue UV-induced immunosuppression in a contact hypersensitivity study. In a chronic UV photocarcinogenesis protocol, topical NPS-2143 reduced squamous cell carcinomas for only up to 24 weeks (p < 0.02) but had no other effect on skin tumour development. In human keratinocytes, 1,25D, which protected mice from UV-induced skin tumours, significantly reduced UV-upregulated p-CREB expression (p < 0.01), a potential early anti-tumour marker, while NPS-2143 had no effect. This result, together with the failure to reduce UV-induced immunosuppression, may explain why the reduction in UV-DNA damage in mice with NPS-2143 was not sufficient to inhibit skin tumour formation.

Protective effects of 1,25 dihydroxyvitamin D3 and its analogs on ultraviolet radiation-induced oxidative stress: a review

The active vitamin D compound, 1,25-dihydroxyvitamin D3 (1,25D) is produced in skin cells following exposure to ultraviolet radiation (UV) from the sun. However, there are many harmful effects of UV which include DNA damage caused by direct absorption of UV, as well as that caused indirectly via UV-induced reactive oxygen species (ROS). Interestingly, 1,25D and analogs have been shown to reduce both direct and indirect UV-induced DNA damage in skin cells. This was accompanied by reductions in ROS and in nitric oxide products with 1,25D following UV. Moreover, following acute UV exposure, 1,25D has been demonstrated to increase p53 levels in skin, which would presumably allow for repair of cells with damaged DNA, or apoptosis of cells with irreparably damaged DNA. Previous studies have also shown that p53 reduces intracellular ROS. Furthermore, 1,25D has been shown to induce metallothioneins, which are potent free radical scavengers. In addition to these protective effects, 1,25D has been demonstrated to inhibit stress-activated c-Jun N-terminal kinases following UV exposure, and to increase levels of the stress-induced protein heme oxygenase-1 in a model of oxidative stress. Herein, we discuss the protective effects of 1,25D and analogs in the context of UV, oxidative stress and skin cancer.

Apple Extract (Malus sp.) and Rutin as Photochemopreventive Agents: Evaluation of Ultraviolet B-Induced Alterations on Skin Biopsies and Tissue-Engineered Skin

The skin is exposed to the solar ultraviolet B (UVB) radiation, which leads to the formation of several types of skin damage responsible for cancer initiation and aging. Malus sp. is a genus of apples, which are a good source of polyphenolic compounds. Malus sp. and more precisely one of its components, rutin, have preventive effects on many diseases caused by reactive oxygen species. In addition, previous studies have suggested the topical usage of the extract as a cosmetic product to prevent skin damage caused by oxidative stress. In this study, we evaluated the efficacy of two topical formulations containing 1.25% of Malus sp. extract and the equivalent amount of rutin (0.75%). The photochemopreventive effect was assessed on two three-dimensional (3D) skin models, that is, ex vivo skin explants and 3D tissue-engineered skin to compare the models. Both formulations protected against the UVB-induced increase in sunburn cell formation, as well as caspase-3 activation and cyclobutane pyrimidine dimer formation in both skin models. Furthermore, the formulations inhibited the lipid peroxidation and the metalloproteinase formation induced by UVB radiation. The tissue-engineered skins and the skin explants provided effective tools to assess the UVB-induced damages. These results support use of the Malus sp. extract and rutin as skin photochemopreventive agents for topical application.

N-Terminal Acetylation and C-Terminal Amidation of Spirulina platensis-Derived Hexapeptide: Anti-Photoaging Activity and Proteomic Analysis

Ultraviolet (UV) irradiation is a potent inducer for skin photoaging. This paper investigated the anti-photoaging effects of the acetylated and amidated hexapeptide (AAH), originally identified from Spirulina platensis, in (Ultraviolet B) UVB-irradiated Human immortalized keratinocytes (Hacats) and mice. The results demonstrated that AAH had much lower toxicity on Hacats than the positive matrixyl (81.52% vs. 5.32%). Moreover, AAH reduced MDA content by 49%; increased SOD, CAT, and GSH-Px activities by 103%, 49%, and 116%, respectively; decreased MMP-1 and MMP-3 expressions by 27% and 29%, respectively, compared to UVB-irradiated mice. Employing isobaric tags for relative and absolute quantitation (iTRAQ)-based proteomics, 60 differential proteins were identified, and major metabolic pathways were determined. Network analysis indicated that these differential proteins were mapped into an interaction network composed of two core sub-networks. Collectively, AAH is protective against UVB-induced skin photoaging and has potential application in skin care cosmetics.

Spironolactone Depletes the XPB Protein and Inhibits DNA Damage Responses in UVB-Irradiated Human Skin

UVB wavelengths of light induce the formation of photoproducts in genomic DNA that are potentially mutagenic and detrimental to epidermal cell function. The mineralocorticoid and androgen receptor antagonist spironolactone (SP) was recently identified as an inhibitor of UV photoproduct removal in human cancer cells in vitro via its ability to promote the rapid proteolytic degradation of the DNA repair protein XPB. Using normal human keratinocytes in vitro and skin explants ex vivo, we found that SP rapidly depleted XPB protein in both systems and abrogated two major responses to UVB-induced DNA damage, including the removal of UV photoproducts from genomic DNA and the activation of ATR/ATM DNA damage kinase signaling. These effects were also correlated with both mutagenesis and a predisposition to UVB-induced cell death but were unique to SP, because neither the SP metabolites canrenone and 7α-thiomethylspironolactone nor the more specific mineralocorticoid receptor antagonist eplerenone affected XPB protein levels or the UVB response. Our findings provide an approach for studying XPB and its roles in the UVB DNA damage response in human skin ex vivo and indicate that SP may increase UVB mutagenesis and skin cancer risk in certain individuals.

Study of the influence of substrate and spectrophotometer characteristics on the in vitro measurement of sunscreens efficiency

All the methods used for the in vitro measurement of the SPF, the universal indicator of sunscreens efficiency, rely on a spectrophotometric analysis. What can vary about the experimental protocol used is mainly the substrate and the type of spectrophotometer chosen. We decided to work with polymethylmetacrylate plates that we analyzed using two spectrophotometers equipped with integrating spheres, the UV1000S and the UV2000 apparatus. Two marketed products were such tested, after spreading 2 mg/cm² on the plates, using one apparatus after another. We applied a non-parametric Wilcoxon test for paired data to the measures realized on 10 plates (as we systematically used the 2 apparatus), in order to compare the series of measures obtained with the two machines. This way, we were able to show a significant difference between the SPF values respectively obtained with the UV1000S and the UV2000 spectrophotometers. This difference could be explained by the decrease of the stray light in the case of the UV2000 apparatus.

Topical application of superoxide dismutase mediated by HIV-TAT peptide attenuates UVB-induced damages in human skin

The purpose of this study was to evaluate whether topical application of superoxide dismutase with cell penetrating peptide (HIV-TAT) could protect against skin damage induced by UVB irradiation in humans. The permeability through stratum corneum of large proteins linked to TAT peptide was firstly confirmed by confocal microscopy and tape stripping. Ten healthy volunteers with either Fitzpatrick skin type II or III were recruited in this clinical study. TAT-SOD (300units/cm(2)) and vehicle cream were applied on two symmetric areas of both inner upper arms 1h prior to UVB irradiation. After one hour of pretreatment, subjects received 10 incremental doses of UVB on pretreated areas. 24h later, erythema, blood flow and apoptotic cells were measured. Pretreatment with TAT-SOD 1h prior to UVB radiation promoted a mean minimal erythema dose (MED) increase of 36.6±18.4% (p=0.013<0.05. n=10) compared to vehicle control. The median blood flow values of all subjects following 2 and 3-MED of UVB were 107.8±51.0units and 239.5±88.0units respectively, which account for 26% and 25% decrease with respect to vehicle groups. These data suggest that TAT-SOD significantly suppresses UVB induced erythema formation and blood flow rise. Furthermore, pretreatment with TAT-SOD 1h prior to 2-MED of UVB irradiation reduced the apoptotic sunburn cell formation by 47.6±8.6% (p<0.0001) in all subjects. Evaluating results generated from all measurements, we conclude that topical application of TAT-SOD significantly attenuates UVB-induced skin damage in man. These biological effects of TAT-SOD are probably mediated via its free radical scavenging properties, clearly differentiating it from other physical sunscreen agents.

Towards a Better Understanding of the Effects of UV on Atlantic Walruses, Odobenus rosmarus rosmarus: A Study Combining Histological Data with Local Ecological Knowledge

Walruses, Odobenus rosmarus, play a key role in the Arctic ecosystem, including northern Indigenous communities, which are reliant upon walruses for aspects of their diet and culture. However, walruses face varied environmental threats including rising sea-water temperatures and decreasing ice cover. An underappreciated threat may be the large amount of solar ultraviolet radiation (UV) that continues to reach the Arctic as a result of ozone loss. UV has been shown to negatively affect whales. Like whales, walrus skin is unprotected by fur, but in contrast, walruses spend long periods of time hauled-out on land. In this study, we combined the results of histological analyses of skin sections from five Atlantic walruses, Odobenus rosmarus rosmarus, collected in Nunavik (Northern Quebec, Canada) with qualitative data obtained through the interviews of 33 local walrus hunters and Inuit Elders. Histological analyses allowed us to explore UV-induced cellular lesions and interviews with experienced walrus hunters and Elders helped us to study the incidences and temporal changes of UV-induced gross lesions in walruses. At the microscopic scale, we detected a range of skin abnormalities consistent with UV damage. However, currently such UV effects do not seem to be widely observed at the whole-animal level (i.e., absence of skin blistering, erythema, eye cataract) by individuals interviewed. Although walruses may experience skin damage under normal everyday UV exposure, the long-term data from local walrus hunters and Inuit Elders did not report a relation between the increased sun radiation secondary to ozone loss and walrus health.

DNA photoprotection conveyed by sunscreen

INTRODUCTION

Skin photoaging is the consequence of solar UV exposure, and DNA damage is an important part of this process. The objective of the current work was to demonstrate that in vitro skin models can be utilized to confirm that DNA damage protection is provided by sunscreens.

METHODS

Skin equivalents were exposed to full-spectrum UV light administered with a standard research solar simulator with and without pre-application of sunscreen. Cyclopyrimidine dimer (CPD) and sunburn cell (SBC) formation as well as CPD quantitation were evaluated to determine DNA damage protection provided by the sunscreen.

RESULTS

Marked decreases in both CPDs and SBCs were observed when sunscreen was applied prior to UV exposure.

CONCLUSIONS

Sunscreen application prior to full-spectrum solar UV exposure protects DNA from photodamage measured by CPD and SBC formation. This can be expected to lessen the risk of photoaging and malignant transformation.

Skin responses to micro scale field size of solar-simulated radiation--preliminary evaluation by reflectance confocal microscopy in vivo

Erythema and pigment responses of human skin following an acute exposure to ultraviolet radiation (UVR) are frequently used to determine the photosensitivity of the skin. In this study we investigated the responses of the skin to a micro-scale area of UVR exposure (MiR) and compared the responses to a macro-scale area of exposure (MaR). Ten human volunteers were tested with solar-simulated radiation on their upper arm or back using a beam size of 8 mm and 0.2 mm in diameter. The fluence required to produce a minimally perceptible erythema (MED) using the MiR was found to be higher than that for the MaR. The erythema response extended beyond the exposed area and this became pronounced when the beam size was microscopic. Reflectance confocal microscopy in vivo revealed that MiR induced cellular alterations within a confined area of smaller dimensions than the area of exposure. Pigment responses were confined within the areas of cellular damage. The erythema expression of exposed skin recovered faster for the sites receiving MiR even when the applied fluence was higher than the MED for the MaR. Through the use of MiR we were able to visualize spatially dissimilar skin responses of erythema and pigmentation suggesting different cellular mechanisms.

Ribosomal stress, p53 activation and the tanning response

Melanocytes (MC) sit along the epidermal basal layer, largely quiescent except for constitutive melanin production. They are usually only activated after sun exposure. The recent paper by McGowan et al. (1) describes a novel mechanism by which melanocytes are induced to proliferate upon p53 activation in adjacent keratinocytes (KC). In this study, small subunit ribosomal protein mutations cause a dramatic activation of p53 that we propose mimics important aspects of the skin sunburn response after ultraviolet radiation (UVR) exposure. McGowan et al. show that the phenotype of their hyperpigmented mouse mutants results from p53-dependent upregulation of KITLG, a cytokine that binds to the KIT receptor on melanocytes and influences melanin synthesis, melanocyte proliferation, and dictates MC localization at the dermo-epidermal junction. These findings extend our knowledge about skin stress responses, in particular, how p53 activity in keratinocytes is central to the regulation of melanocyte behaviour.

The roles of autophagy and apoptosis in burn wound progression in rats

Understanding the role of cell death in burn wound progression is crucial for giving appropriate diagnoses and designing therapy regimens for burn patients. A well-described and reliable "comb burns model" was employed to evaluate the roles of autophagy and apoptosis in burn wound progression at 2 h, 6 h, 12 h, 24 h, and 48 h post-burn in a rat model. Immunohistochemistry (IHC) results showed that autophagy was detectable in hair follicle epithelium at 2 h post-burn, peaked at 12 h post-burn, then declined. Conversely, apoptosis was mainly located in the stratum epidermis and took place at low levels until 6 h post-burn, at which point it slowly increased. Bcl-2 and Bax, which are regulators of both processes, showed protein expression level patterns that were consistent with the IHC results. This study of autophagy in burn wound tissue progression represents a conceptual expansion of cell death in burn wounds. Based on these results, we suggest that different treatments should be performed on a specific post-burn time course depending on the most prevalent type of cell death occurring at that time.

Potent anticancer activity of cystine-based dipeptides and their interaction with serum albumins

BACKGROUND

Cancer is a severe threat to the human society. In the scientific community worldwide cancer remains a big challenge as there are no remedies as of now. Cancer is quite complicated as it involves multiple signalling pathways and it may be caused by genetic disorders. Various natural products and synthetic molecules have been designed to prevent cell proliferation. Peptide-based anticancer drugs, however, are not explored properly. Though peptides have their inherent proteolytic instability, they could act as anticancer agents.

RESULTS

In this present communication a suitably protected cystine based dipeptide and its deprotected form have been synthesized. Potent anticancer activities were confirmed by MTT assay (a laboratory test and a standard colorimetric assay, which measures changes in colour, for measuring cellular proliferation and phase contrast images. The IC50 value, a measure of the effectiveness of a compound in inhibiting biological or biochemical function, of these compounds ranges in the sub-micromolar level. The binding interactions with serum albumins (HSA and BSA) were performed with all these molecules and all of them show very strong binding at sub-micromolar concentration.

CONCLUSIONS

This study suggested that the cystine-based dipeptides were potential anticancer agents. These peptides also showed very good binding with major carrier proteins of blood, the serum albumins. We are currently working on determining the detailed mechanism of anticancer activity of these molecules.

Vitamin D and death by sunshine

Exposure to sunlight is the major cause of skin cancer. Ultraviolet radiation (UV) from the sun causes damage to DNA by direct absorption and can cause skin cell death. UV also causes production of reactive oxygen species that may interact with DNA to indirectly cause oxidative DNA damage. UV increases accumulation of p53 in skin cells, which upregulates repair genes but promotes death of irreparably damaged cells. A benefit of sunlight is vitamin D, which is formed following exposure of 7-dehydrocholesterol in skin cells to UV. The relatively inert vitamin D is metabolized to various biologically active compounds, including 1,25-dihydroxyvitamin D3. Therapeutic use of vitamin D compounds has proven beneficial in several cancer types, but more recently these compounds have been shown to prevent UV-induced cell death and DNA damage in human skin cells. Here, we discuss the effects of vitamin D compounds in skin cells that have been exposed to UV. Specifically, we examine the various signaling pathways involved in the vitamin D-induced protection of skin cells from UV.

Ultraviolet radiation effects on the proteome of skin cells

Proteomic studies to date have had limited use as an investigative tool in the skin's response to UV radiation. These studies used cell lines and reconstructed skin and have shown evidence of cell injury with oxidative damage and stress induced heat shock proteins. Others changes included altered cytokeratin and cytoskeletal proteins with enhanced expression of TRIM29 as the keratinocytes regenerate. The associated DNA repair requires polη, Rad18/Rad16 and Rev1. In the whole animal these events would be associated with inflammation, remodelling of the epidermis and modulation of the immune response. Longer term changes include ageing and skin cancers such as melanoma, squamous cell carcinoma and basal cell carcinoma. In the future proteomics will be used to explore these important aspects of photobiology. Better characterisation of the proteins involved should lead to a greater understanding of the skin's response to UV radiation.

Photo-oxidation of proteins

Photo-induced damage to proteins occurs via multiple pathways. Direct damage induced by UVB (λ 280-320 nm) and UVA radiation (λ 320-400 nm) is limited to a small number of amino acid residues, principally tryptophan (Trp), tyrosine (Tyr), histidine (His) and disulfide (cystine) residues, with this occurring via both excited state species and radicals. Indirect protein damage can occur via singlet oxygen ((1)O(2)(1)Δ(g)), with this resulting in damage to Trp, Tyr, His, cystine, cysteine (Cys) and methionine (Met) residues. Although initial damage is limited to these residues multiple secondary processes, that occur both during and after radiation exposure, can result in damage to other intra- and inter-molecular sites. Secondary damage can arise via radicals (e.g. Trp, Tyr and Cys radicals), from reactive intermediates generated by (1)O(2) (e.g. Trp, Tyr and His peroxides) and via molecular reactions of photo-products (e.g. reactive carbonyls). These processes can result in protein fragmentation, aggregation, altered physical and chemical properties (e.g. hydrophobicity and charge) and modulated biological turnover. Accumulating evidence implicates these events in cellular and tissue dysfunction (e.g. apoptosis, necrosis and altered cell signaling), and multiple human pathologies.

Omega-3 polyunsaturated fatty acids: photoprotective macronutrients

Ultraviolet radiation (UVR) in sunlight has deleterious effects on skin, while behavioural changes have resulted in people gaining more sun exposure. The clinical impact includes a year-on-year increase in skin cancer incidence, and topical sunscreens alone provide an inadequate measure to combat overexposure to UVR. Novel methods of photoprotection are being targeted as additional measures, with growing interest in the potential for systemic photoprotection through naturally sourced nutrients. Omega-3 polyunsaturated fatty acids (n-3 PUFA) are promising candidates, showing potential to protect the skin from UVR injury through a range of mechanisms. In this review, we discuss the biological actions of n-3 PUFA in the context of skin protection from acute and chronic UVR overexposure and describe how emerging new technologies such as nutrigenomics and lipidomics assist our understanding of the contribution of such nutrients to skin health.

Different oxidative stress response in keratinocytes and fibroblasts of reconstructed skin exposed to non extreme daily-ultraviolet radiation

Experiments characterizing the biological effects of sun exposure have usually involved solar simulators. However, they addressed the worst case scenario i.e. zenithal sun, rarely found in common outdoor activities. A non-extreme ultraviolet radiation (UV) spectrum referred as "daily UV radiation" (DUVR) with a higher UVA (320-400 nm) to UVB (280-320 nm) irradiance ratio has therefore been defined. In this study, the biological impact of an acute exposure to low physiological doses of DUVR (corresponding to 10 and 20% of the dose received per day in Paris mid-April) on a 3 dimensional reconstructed skin model, was analysed. In such conditions, epidermal and dermal morphological alterations could only be detected after the highest dose of DUVR. We then focused on oxidative stress response induced by DUVR, by analyzing the modulation of mRNA level of 24 markers in parallel in fibroblasts and keratinocytes. DUVR significantly modulated mRNA levels of these markers in both cell types. A cell type differential response was noticed: it was faster in fibroblasts, with a majority of inductions and high levels of modulation in contrast to keratinocyte response. Our results thus revealed a higher sensitivity in response to oxidative stress of dermal fibroblasts although located deeper in the skin, giving new insights into the skin biological events occurring in everyday UV exposure.

Induction of the skin endogenous protective mitochondrial MnSOD by Vitreoscilla filiformis extract

Vitreoscilla filiformis (Vf), a filamentous bacteria living in fresh water is thought to contribute to the observed beneficial effects of Spa water on skin. An active fraction obtained from a Vf biomass was evaluated for its ability to modulate mRNA expression in cultured skin cells. cDNA array analysis was conducted first using a customized membrane including 1176 selected and fully identified genes involved in skin physiology and homeostasis then the newly developed full genome U133 plus 2.0 GeneChip from Affymetrix. The mitochondrial protective manganese superoxide dismutase (MnSOD/SOD-2) was identified as a preferentially induced mRNA target in both normal human dermal fibroblasts and keratinocytes. Induction at the transcriptional level in both cell types was confirmed using quantitative real time/polymerase chain reaction and a kinetic analysis revealed a maximal increase in mRNA expression 20 h after stimulation with Vf extract (Vfe). Using immunofluorescent (fluorescent cell sorter) analysis, an induction of MnSOD protein in both normal human dermal skin fibroblasts (x1.6; P < 0.01) and epidermal keratinocytes (x1.4; P < 0.01) was confirmed. As MnSOD is a major inducible free-radical scavenger in skin, these results suggest that the Vfe could induce skin cells to produce their own endogenous protective defences in vivo against both exogenous environmental stressors such as UV irradiation or microflora as well as to combat endogenous sources of deleterious free radicals involved in skin ageing. Finally, in order to confirm the in vivo potential of this original extract in human, we evaluated its protective activity vs. placebo on the generation of sunburn cells in epidermis under UVB stress. As expected from in vitro profiling, Vfe was indeed found to significantly inhibit the appearance of sunburn cells in UVB-exposed areas, a signature of skin alteration which has been suggested to be linked to a defect in MnSOD protective activity. Altogether, those data suggest that the combination of a suitable protective UV filter together with this bioactive Vfe might improve skin protection through complementary pathways.

Application of the concept of relative photomutagenic potencies to selected furocoumarins in V79 cells

Furocoumarins are phototoxic and photomutagenic natural plant constituents found in many medicinal plants and food items. Since plants contain mixtures of several furocoumarins, there is a need for a comparative risk assessment of a large number of furocoumarins. Previously, we have introduced the concept of relative Photomutagenicity Equivalency Factors (PMEFs) derived from the slope of the concentration-response curve of photomutagenicity of individual furocoumarins in V79 cells using the HPRT mutation assay in the presence of UVA irradiation at 125mJ/cm(2). Here we have applied this method to the furocoumarins bergamottin, isopimpinellin and psoralen using 5-methoxypsoralen (5-MOP) as a reference compound with a PMEF of 1.0. We found that neither bergamottin nor isopimpinellin, two furocoumarins abundant in plants, food etc., exerted any significant photomutagenicity while psoralen was clearly photomutagenic with a PMEF of 0.36. Similarly, isopimpinellin was not phototoxic in V79 cells, while bergamottin showed some cytotoxicity which, however, was completely independent of UVA irradiation. Only psoralen was photocytotoxic showing a similar concentration-response relationship for photomutagenicity, and for photocytotoxicity (at 72h after irradiation). Data from the micronucleus assay for DNA damage at 20h after irradiation were in complete agreement with the HPRT mutation data. Our findings indicate that individual furocoumarins differ enormously in their photomutagenic potency, and that a specific toxicological risk assessment is required for each furocoumarin instead of an assessment based on the sum of furocoumarins in a given sample.

The effects of topically applied glycolic acid and salicylic acid on ultraviolet radiation-induced erythema, DNA damage and sunburn cell formation in human skin

BACKGROUND

alpha-Hydroxy acids (alphaHAs) are reported to reduce signs of aging in the skin and are widely used cosmetic ingredients. Several studies suggest that alphaHA can increase the sensitivity of skin to ultraviolet radiation. More recently, beta-hydroxy acids (betaHAs), or combinations of alphaHA and betaHA have also been incorporated into antiaging skin care products. Concerns have also arisen about increased sensitivity to ultraviolet radiation following use of skin care products containing beta-HA.

OBJECTIVE

To determine whether topical treatment with glycolic acid, a representative alphaHA, or with salicylic acid, a betaHA, modifies the short-term effects of solar simulated radiation (SSR) in human skin.

METHODS

Fourteen subjects participated in this study. Three of the four test sites on the mid-back of each subject were treated daily Monday-Friday, for a total of 3.5 weeks, with glycolic acid (10%), salicylic acid (2%), or vehicle (control). The fourth site received no treatment. After the last treatment, each site was exposed to SSR, and shave biopsies from all four sites were obtained. The endpoints evaluated in this study were erythema (assessed visually and instrumentally), DNA damage and sunburn cell formation.

RESULTS

Treatment with glycolic acid resulted in increased sensitivity of human skin to SSR, measured as an increase in erythema, DNA damage and sunburn cell formation. Salicylic acid did not produce significant changes in any of these biomarkers.

CONCLUSIONS

Short-term topical application of glycolic acid in a cosmetic formulation increased the sensitivity of human skin to SSR, while a comparable treatment with salicylic acid did not.

Secondary necrosis in multicellular animals: an outcome of apoptosis with pathogenic implications

In metazoans apoptosis is a major physiological process of cell elimination during development and in tissue homeostasis and can be involved in pathological situations. In vitro, apoptosis proceeds through an execution phase during which cell dismantling is initiated, with or without fragmentation into apoptotic bodies, but with maintenance of a near-to-intact cytoplasmic membrane, followed by a transition to a necrotic cell elimination traditionally called “secondary necrosis”. Secondary necrosis involves activation of self-hydrolytic enzymes, and swelling of the cell or of the apoptotic bodies, generalized and irreparable damage to the cytoplasmic membrane, and culminates with cell disruption. In vivo, under normal conditions, the elimination of apoptosing cells or apoptotic bodies is by removal through engulfment by scavengers prompted by the exposure of engulfment signals during the execution phase of apoptosis; if this removal fails progression to secondary necrosis ensues as in the in vitro situation. In vivo secondary necrosis occurs when massive apoptosis overwhelms the available scavenging capacity, or when the scavenger mechanism is directly impaired, and may result in leakage of the cell contents with induction of tissue injury and inflammatory and autoimmune responses. Several disorders where secondary necrosis has been implicated as a pathogenic mechanism will be reviewed.

Radiation Sources Providing Increased UVA/UVB Ratios Attenuate the Apoptotic Effects of the UVB Waveband UVA-Dose-Dependently in Hairless Mouse Skin

UV radiation-induced epidermal apoptotic sunburn cells provide a mechanism for eliminating cells with irreparable DNA damage. The UVB (290-320 nm) waveband is mainly responsible, but the role of UVA (320-400 nm) is less clear, and possible waveband interactions have not been examined. Recent studies in mice reveal a protective role for UVA against UVB-induced inflammation and immunosuppression, mediated via cutaneous heme oxygenase (HO). As HO has antiapoptotic properties in other tissues, this study examines the effect of UVA/UVB waveband interaction on apoptosis in the Skh:hr-1 hairless mouse epidermis. Apoptosis was assessed by sunburn cell number, caspase-3-positive cell number, and degree of DNA fragmentation, in mice exposed to radiation sources providing a constant UVB dose with increasing proportions of UVA. The results indicated that as the UVA/UVB ratio was increased, both the sunburn cell and caspase-3-positive cell number decreased, and the degree of DNA fragmentation was reduced. Treatment of mice with the HO inhibitor, tin protoporphyrin-IX, markedly reduced the UVA antiapoptotic effect, confirming a major role for HO. The observations suggest that UVA reduces UVB-induced DNA damage, and may therefore have anti-photocarcinogenic properties that could be harnessed for better photoprotection in humans.

A keratinocyte's course of life

An adequate permeability barrier function of the mammalian epidermis is guaranteed by the characteristic architecture of the stratum corneum. This uppermost layer consists of a highly organized extracellular lipid compartment which is tightly joined to the corneocytes. The generation of the extracellular lipid compartment and the transformation of the keratinocytes into corneocytes are the main features of epidermal differentiation. However, equally important is the continuous renewal of the stratum corneum, which is insured by a careful balance between the replenishment of new keratinocytes from the proliferating basal layer, and the well-orchestrated loss of the most superficial cells after the so-called 'epidermal programmed cell death'. In this overview, the complete life of keratinocytes is described, from the proliferative organization to the process of desquamation.

Quantitative Analysis of Radiation-Induced DNA Break Repair in a Cultured Oral Mucosal Model

Oral mucositis is a major side effect of radiation therapy. Development of strategies for reduction of this problem calls for quantitative models. The goal of the present study was to test the feasibility of detecting double-strand breaks (DSBs) and DSB repair proteins upon radiation of mucosa in a 3-dimensional culture system using morphology and immunohistochemistry. Human oral keratinocytes and fibroblasts were seeded onto and into an acellular dermal carrier to produce a cultured mucosal substitute (CMS). CMSs were gamma-irradiated with 0, 2, and 12 Gy. One group received 4 Gy through 2 Gy fractions with a 24-h interval. Radiation-induced damage was quantified using hematoxylin and eosin (H&E). DSBs and DSB repair proteins were visualized and quantified using antibodies against P53 binding protein 1 (53BP1), MRE11, and RAD51. As in cell culture, CMSs showed intranuclear loci of damage and repair, mostly in the proliferative basal cell layers. Maximum percentages of damaged basal layer keratinocytes were 54.8% using H&E (12 Gy) up to 78.9% (12 Gy) for 53BP1. This study shows the feasibility of DNA repair markers to quantify radiation damage. This is an important step forward in the study of mucositis and the development of treatment and prevention strategies, proving once more the power and clinical importance of tissue engineering.

Acute effects of UVR on human eyes and skin

Solar UVR ( approximately 295-400 nm) has acute clinical effects on the eyes and the skin. The only effect on the eye is inflammation of the cornea (photokeratitis), which is caused by UVB (and non-solar UVC) and resolves without long-term consequences within 48 h. The effects on the skin are more extensive and include sunburn (inflammation), tanning and immunosuppression for which UVB is mainly responsible. Tanning is modestly photoprotective against further acute UVR damage. Skin colour is also transiently changed by UVA-dependent immediate pigment darkening, the function of which is unknown. Skin type determines sensitivity to the acute and chronic effects of UVR on the skin. Some of the photochemical events that initiate acute effects are also related to skin cancer. Solar UVB is also responsible for the synthesis of vitamin D.

The Differentiation-dependent Desmosomal Cadherin Desmoglein 1 Is a Novel Caspase-3 Target That Regulates Apoptosis in Keratinocytes

Although a number of cell adhesion proteins have been identified as caspase substrates, the potential role of differentiation-specific desmosomal cadherins during apoptosis has not been examined. Here, we demonstrate that UV-induced caspase cleavage of the human desmoglein 1 cytoplasmic tail results in distinct 17- and 140- kDa products, whereas metalloproteinase-dependent shedding of the extracellular adhesion domain generates a 75-kDa product. In vitro studies identify caspase-3 as the preferred enzyme that cleaves desmoglein 1 within its unique repeating unit domain at aspartic acid 888, part of a consensus sequence not conserved among the other desmosomal cadherins. Apoptotic processing leads to decreased cell surface expression of desmoglein 1 and re-localization of its C terminus diffusely throughout the cytoplasm over a time course comparable with the processing of other desmosomal proteins and cytoplasmic keratins. Importantly, whereas classic cadherins have been reported to promote cell survival, short hairpin RNA-mediated suppression of desmoglein 1 in differentiated keratinocytes protected cells from UV-induced apoptosis. Collectively, our results identify desmoglein 1 as a novel caspase and metalloproteinase substrate whose cleavage likely contributes to the dismantling of desmosomes during keratinocyte apoptosis and also reveal desmoglein 1 as a previously unrecognized regulator of apoptosis in keratinocytes.

Apoptosis induced by ultraviolet B in HPV-immortalized human keratinocytes requires caspase-9 and is death receptor independent

Ultraviolet B (UVB) induces both apoptosis and skin cancer. We found that human keratinocytes (KC) immortalized by Human Papillomavirus (HPV)16 E6/E7 were sensitized to UVB-induced apoptosis, possibly representing a transient regression-prone precancerous stage equivalent to actinic keratosis. To further examine which caspases are apical and essential, we utilized retroviral constructs expressing dominant-negative caspase-9 (caspase-9-DN) or Fas-associated protein with death domain (FADD)-DN as well as caspase inhibitor peptides. Caspase-9-DN and zLEHD-fmk both suppressed caspase-9, -3, and -8 activity after UVB exposure, as well as proteolytic processing of procaspase-3 into its active form, DNA fragmentation factor 45 cleavage, and internucleosomal DNA fragmentation. By contrast, stable expression of FADD-DN in HPV-immortalized KC did not inhibit UVB-induced activation of caspases-9, -3, and -8 nor downstream apoptotic events, although inhibition of caspase-8 with zIETD-fmk attenuated apoptosis. This study indicates that caspase-9 activation is upstream of caspases-3 and -8 and that UVB-induced apoptosis in HPV-immortalized human KC is death receptor (DR) independent and requires both caspase-9 upstream and caspase-8 downstream for maximal apoptosis. These studies further indicate that cell type as well as transformation state determine the sensitivity and mode of cell death (DR vs. mitochondrial apoptotic pathways) in response to UVB and explain the high regression rates of premalignant lesions.

Melanin acts as a potent UVB photosensitizer to cause an atypical mode of cell death in murine skin

Melanin protects the skin against DNA damage induced by direct absorption of sunlight's UV radiation. Yet, irradiating melanin in vitro or in cultured cells also generates active oxygen species such as superoxide, which can indirectly induce oxidative base lesions and DNA strand breaks. This photosensitization is greater for pheomelanin (yellow and red melanin) than for eumelanin (brown and black). The in vivo photosensitizing ability of melanin is unknown. We used congenic mice of black, yellow, and albino coat colors to investigate the induction of DNA lesions and apoptosis after exposure to predominantly UVB (280-320 nm) or UVA (320-400 nm) radiation. Cyclobutane pyrimidine dimers induced by direct UVB absorption were equal in all three strains, as was apoptosis measured as sunburn cells or as keratinocytes containing active caspase-3. However, terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling (TUNEL)-positive cells were approximately 3-fold more frequent in black and yellow mice after UVB or UVA irradiation than in albino. In epidermal sheets, TUNEL-positive cells lined the upper portion of the hair follicle, consistent with UV-induced photosensitization by melanin in the hair shaft. Because the concentration of eumelanin in black mice was three times that of pheomelanin in yellow mice, pheomelanin had 3-fold greater specific activity. We conclude that UV-irradiated melanin, particularly pheomelanin, photosensitizes adjacent cells to caspase-3 independent apoptosis, and this occurs at a frequency greater than the apoptosis induced by direct DNA absorption of UV. Melanin-induced apoptosis may contribute to the increased sensitivity of individuals with blonde and red hair to sunburn and skin cancer.