Protein oxidation in human stratum corneum: susceptibility of keratins to oxidation in vitro and presence of a keratin oxidation gradient in vivo

Tight junctions in the stratum corneum explain spatial differences in corneodesmosome degradation

To maintain stratum corneum integrity while simultaneously desquamating at a steady rate, degradation of corneodesmosomes must proceed in a controlled manner. It is unknown why corneodesmosomes are present only at the cell periphery in the upper stratum corneum. To explore this, we studied distributions of three major corneodesmosomal components, corneodesmosin, desmoglein 1 and desmocollin 1 in normal adult human epidermis. Immunofluorescent microscopy studies of skin surface corneocytes detected all three components only at the cell edges. Immunoelectron microscopy revealed selective loss of these components at the central areas starting from the deep cornified layers. We hypothesized that tight junctions (TJs) formed in the superficial granular layer may prevent protease access by functioning as a barrier between the peripheral and the central intercellular spaces in the stratum corneum. Ultrastructural examination demonstrated TJs up to the junctions between the seventh and the eighth deepest cornified layers. Immunoelectron microscopy also detected clusters of occludin and claudin-1 immunolabels at the cell periphery, and kallikrein 7 immunolabels outside of TJs in the lower cornified layers. With colloidal lanthanum nitrate perfusion assay of stripped stratum corneum, the tracer was excluded from TJ domains. Taken together, we propose that TJs inhibit access of proteases to the peripheral corneodesmosomes forming the structural basis for the basket-weave-like appearance of the stratum corneum.

The malondialdehyde-derived fluorophore DHP-lysine is a potent sensitizer of UVA-induced photooxidative stress in human skin cells

Light-driven electron and energy transfer involving non-DNA skin chromophores as endogenous photosensitizers induces oxidative stress in UVA-exposed human skin, a process relevant to photoaging and photocarcinogenesis. Malondialdehyde is an electrophilic dicarbonyl-species derived from membrane lipid peroxidation. Here, we present experimental evidence suggesting that the malondialdehyde-derived protein epitope dihydropyridine (DHP)-lysine is a potent endogenous UVA-photosensitizer of human skin cells. Immunohistochemical analysis revealed the abundant occurrence of malondialdehyde-derived and DHP-lysine epitopes in human skin. Using the chemically protected dihydropyridine-derivative (2S)-Boc-2-amino-6-(3,5-diformyl-4-methyl-4H-pyridin-1-yl)-hexanoic acid-t-butylester as a model of peptide-bound DHP-lysine, photodynamic inhibition of proliferation and induction of cell death were observed in human skin Hs27 fibroblasts as well as primary and HaCaT keratinocytes exposed to the combined action of UVA and DHP-lysine. DHP-lysine photosensitization induced intracellular oxidative stress, p38 MAPkinase activation, and upregulation of heme oxygenase-1 expression. Consistent with UVA-driven ROS formation from DHP-lysine, formation of superoxide, hydrogen peroxide, and singlet oxygen was detected in chemical assays, but little protection was achieved using SOD or catalase during cellular photosensitization. In contrast, inclusion of NaN(3) completely abolished DHP-photosensitization. Taken together, these data demonstrate photodynamic activity of DHP-lysine and support the hypothesis that malondialdehyde-derived protein-epitopes may function as endogenous sensitizers of UVA-induced oxidative stress in human skin.

The effect of 2% niacinamide on facial sebum production

BACKGROUND

The presence of sebum on the face is responsible for both facial shine and the formation of comedonal and inflammatory acne lesions. Sebum control is a goal of many OTC skin care products; however, most currently available products function by absorbing sebum from the face rather than modulating its production.

OBJECTIVE

To demonstrate the effect of topical 2% niacinamide on sebum excretion rates and casual sebum production in Oriental and Caucasian populations.

METHODS

Separate clinical trials were conducted in both Japan and the USA to evaluate the effect of topical 2% niacinamide in different ethnic groups. A total of 100 Japanese subjects were enrolled in a double-blind, placebo-controlled comparison between two independent balanced groups. Fifty subjects applied the 2% niacinamide moisturizer to the face for 4 weeks and 50 subjects used a placebo moisturizer for 4 weeks, with sebum excretion rate (SER) measurements taken at baseline, week 2, and week 4. In addition, 30 Caucasian subjects were enrolled in a randomized split-face study for 6 weeks with SER and casual sebum levels (CSL) measured at baseline, week 3, and week 6.

RESULTS

The results of the Japanese study demonstrated that the SER of the two groups was not significantly different at baseline, but the 2% niacinamide treated group demonstrated significantly lowered SER after 2 and 4 weeks of application. The results were somewhat different in the Caucasian study. After 6 weeks of treatment, the CSL was significantly reduced, but the SER was not significantly reduced.

CONCLUSIONS

Topical 2% niacinamide may be effective in lowering the SER in Japanese individuals and CSL in Caucasian individuals.

Increased oxidative stress and antioxidant expression in mouse keratinocytes following exposure to paraquat

Paraquat (1,1'-dimethyl-4,4'-bipyridinium) is a widely used herbicide known to induce skin toxicity. This is thought to be due to oxidative stress resulting from the generation of cytotoxic reactive oxygen intermediates (ROI) during paraquat redox cycling. The skin contains a diverse array of antioxidant enzymes which protect against oxidative stress including superoxide dismutase (SOD), catalase, glutathione peroxidase-1 (GPx-1), heme oxygenase-1 (HO-1), metallothionein-2 (MT-2), and glutathione-S-transferases (GST). In the present studies we compared paraquat redox cycling in primary cultures of undifferentiated and differentiated mouse keratinocytes and determined if this was associated with oxidative stress and altered expression of antioxidant enzymes. We found that paraquat readily undergoes redox cycling in both undifferentiated and differentiated keratinocytes, generating superoxide anion and hydrogen peroxide as well as increased protein oxidation which was greater in differentiated cells. Paraquat treatment also resulted in increased expression of HO-1, Cu,Zn-SOD, catalase, GSTP1, GSTA3 and GSTA4. However, no major differences in expression of these enzymes were evident between undifferentiated and differentiated cells. In contrast, expression of GSTA1-2 was significantly greater in differentiated relative to undifferentiated cells after paraquat treatment. No changes in expression of MT-2, Mn-SOD, GPx-1, GSTM1 or the microsomal GST's mGST1, mGST2 and mGST3, were observed in response to paraquat. These data demonstrate that paraquat induces oxidative stress in keratinocytes leading to increased expression of antioxidant genes. These intracellular proteins may be important in protecting the skin from paraquat-mediated cytotoxicity.

Non-invasive monitoring of oxidative skin stress by ultraweak photon emission measurement. II: biological validation on ultraviolet A-stressed skin

BACKGROUND/PURPOSE

Several physical or chemical environmental stressors generate reactive oxygen species, which trigger oxidation reactions of cells or tissues and thereby induce a correlated ultraweak photon emission (UPE) signal. The present study was designed to qualify and validate UPE measurement following ultraviolet (UV) excitation of porcine and human skin as an analytical method to assess the potency of topical antioxidants in vivo.

METHODS

UPE of porcine skin in vitro and human skin in vivo following excitation with UVA was recorded using sensitive photomultiplier systems. For validation purposes, the effects of variation of extrinsic and intrinsic parameters encompassing skin thickness, humidity, temperature, pH, and composition of the surrounding atmosphere were assessed. Signals were analyzed with regard to overall signal intensity and spectral distribution. In two clinical trials enrolling 20 volunteers each, the effects of topical antioxidant treatment on UVA-induced UPE were validated.

RESULTS

Different stressors encompassing exposition to ozone, UVA irradiation, or even cigarette smoke induced UPE of skin. Critical parameters affecting the quality and quantity of the UPE signal were the spectral composition of the exciting UV light, skin temperature, skin humidity, and the O(2) concentration of the surrounding atmosphere. Generally, UVA-induced UPE decreased with increasing temperature, humidity, and O(2) concentration. Skin pH had no significant effect on UPE with regard to signal quality and quantity over a pH range of 2.8-8.2. In a clinical study UPE measurement following UVA excitation could precisely reflect a dose-dependent antioxidant effect of topically applied vitamin C and alpha-glucosylrutin.

CONCLUSION

Our data indicate that UVA irradiation induces UPE especially in deeper (living) skin layers, where antioxidants must be active in order to interfere with accelerated skin ageing. Based on the clinical data, and with knowledge of modulating external variables, UPE measurement following UV excitation can be qualified as a reliable and valid method for the non-invasive measurement of antioxidant efficacy on the skin.

Non-invasive monitoring of oxidative skin stress by ultraweak photon emission (UPE)-measurement. I: mechanisms of UPE of biological materials

BACKGROUND/PURPOSE

Oxidation of proteins and amino acids is associated with generation of ultraweak photon emission (UPE), which may be used to assess oxidative processes in the skin in a non-invasive way. This first part of a series of reports addresses the physicochemical basis of oxidation-induced UPE in the skin, with a special focus on the contribution of amino acid oxidation.

METHODS

UPE of biological samples and protein/amino acid solutions following oxidation with H(2)O(2) in the presence of Fe(2+) was recorded using a sensitive photomultiplier system. Signals were analyzed with regard to overall signal intensity and spectral distribution.

RESULTS

Increasing concentrations of H(2)O(2) in aqueous bovine serum albumin solutions induced linearly correlated UPE and protein carbonyl compounds, with a substantially higher sensitivity for the measurement of UPE. In single amino acid solutions, strong UPE signals were generated by oxidation from Phe, Trp, His, and Cys, and weak signals from Lys and Thr. Analysis of reaction products by MS revealed high oxidative material turnover for Cys and His, whereas barely detectable oxidative material turnover seems to be sufficient to generate a UPE signal of similar strength from Trp and Phe. Combination of different amino acids did not result in a simple addition of individual oxidation-induced UPE signals, but in interactions ranging from antagonism to clear synergism. Synergism was evident between Trp- and UPE-generating amino acids such as Thr, Cys, and His, with the strongest synergism by far observed between Trp and His. The strikingly different individual UPE spectra of His and Trp, despite being of comparable overall strength, were congruent with a pure Trp UPE spectrum after combining His with Trp in solution, indicating energy transfer between both amino acids. Combination of Trp and DNA, which also gives UPE signals following oxidation, did not result in a synergistically enhanced or antagonized overall UPE signal, but in a simple addition of individual UPE signals.

CONCLUSION

Measurement of UPE could be proven to be a highly sensitive method to assess oxidative processes in biological molecules. The reported data indicate that UPE generated by oxidation stressed skin is mainly due to non-fluorescent photon emission via Trp, whereby Trp acts as an energy receptor from other excited species of oxidation-modified amino acids.

Distinct effects of ultraviolet B light on antioxidant expression in undifferentiated and differentiated mouse keratinocytes

Ultraviolet (UV) B causes oxidative stress, which has been implicated in carcinogenesis. We determined if the sensitivity of keratinocytes to UVB-induced oxidative stress is dependent on their differentiation state. In primary cultures of undifferentiated and differentiated mouse keratinocytes, UVB (25 mJ/cm(2)) stimulated production of reactive oxygen intermediates. This was associated with increased messenger RNA (mRNA) expression of the antioxidant enzymes glutathione peroxidase, heme oxygenase-1 (HO-1) and the glutathione S-transferase (GST), GSTA1-2. The effects of UVB on GSTA1-2 were greater in undifferentiated when compared with differentiated cells. UVB also induced GSTM1, but only in undifferentiated cells. In contrast, UVB reduced expression of manganese superoxide dismutase, metallothionein-2, GSTA3 and microsomal glutathione S-transferase (mGST)3 in both cell types, whereas it had no major effects on catalase, copper-zinc superoxide dismutase, GSTP1, mGST1 or mGST2. Of note, levels of GSTA4 mRNA were 4- to 5-fold greater in differentiated relative to undifferentiated cells. Moreover, whereas GSTA4 was induced by UVB in undifferentiated cells, it was inhibited in differentiated cells. UVB activated p38 and c-jun N-terminal kinase mitogen-activated protein (MAP) kinases in both undifferentiated and differentiated keratinocytes. Whereas inhibition of these kinases blocked UVB-induced HO-1 in both cell types, GSTA1-2 and GST-4 were only suppressed in undifferentiated cells. In differentiated keratinocytes, p38 inhibition also suppressed GSTA1-2. In contrast, MAP kinase inhibition had no major effects on UVB-induced suppression of GSTA4 in differentiated cells. These data indicate that UVB-induced alterations in antioxidant expression are differentiation dependent. Moreover, MAP kinases are critical regulators of this response. Alterations in antioxidants are likely to be important mechanisms for protecting the skin from UVB-induced oxidative stress.

[Sebaceous glands as transporters of vitamin E]

Human sebum is produced by sebaceous glands and reaches the skin surface via secretion through the hair shaft. There is experimental evidence that the sebaceous glands and sebum serve as a transport mechanism taking the lipophilic antioxidant vitamin E from the blood to the skin surface. The highest levels of vitamin E are found in the sebum and in the skin lipid film in sebum-rich areas such as facial skin. Recent studies indicate that daily oral supplementation of moderate doses of alpha-tocopherol for at least 3 weeks leads to significant increases of vitamin E levels in human skin sites with a high density of sebaceous glands, such as the face. Thus, the potential photoprotective and antioxidants effects of oral vitamin E, as well as possibly other antioxidants, are site-dependent. These findings should be considered when designing clinical studies to assess the efficacy of oral antioxidants against oxidative stress in the skin.

Transcription activator structure reveals redox control of a replication initiation reaction

Redox changes are one of the factors that influence cell-cycle progression and that control the processes of cellular proliferation, differentiation, senescence and apoptosis. Proteins regulated through redox-sensitive cysteines have been characterized but specific ‘sulphydryl switches’ in replication proteins remain to be identified. In bovine papillomavirus type-1, DNA replication begins when the viral transcription factor E2 recruits the viral initiator protein E1 to the origin of DNA replication (ori). Here we show that a novel dimerization interface in the E2 transcription activation domain is stabilized by a disulphide bond. Oxidative cross-linking via Cys57 sequesters the interaction surface between E1 and E2, preventing pre-initiation and replication initiation complex formation. Our data demonstrate that as well as a mechanism for regulating DNA binding, redox reactions can control replication by modulating the tertiary structure of critical protein factors using a specific redox sensor.

Effects of UVB radiation on 4-hydroxy-2-trans-nonenal metabolism and toxicity in human keratinocytes

Endogenous lipid peroxidation (LPO)-derived aldehydes accumulate in human skin after photoexposure and contribute to the development of skin cytotoxicity and cancer. This study employed LC-ESI-MS and HPLC-UV-DAD techniques to investigate the effect of UVB radiation on the biotransformation and detoxification of the prototype aldehyde 4-hydroxy-2-trans-nonenal (HNE) using the human keratinocyte cell line (NCTC 2544). In parallel we followed the keratinocytes' cytotoxic response to HNE through morphological analysis and cell viability assay. In UVB-unstressed keratinocytes, even a supraphysiological dose of the aldehyde (200 microM) was rapidly and completely cleared in metabolized form (free and GSH-conjugated metabolites) from the cell, with no signs of cytotoxicity. By contrast, UVB preexposure already at 1 MED (50 mJ/cm2, the minimal erythemal dose in humans) markedly impaired HNE metabolism. After 2 h of incubation, the relative amount of GSH-conjugated adducts dose-dependently dropped from 44% (unirradiated cells) to 22% at 3 MED as a consequence of UVB-induced GSH depletion (no impairment of GST A4.4 nor of G6PD activities was observed). The levels of free metabolites, 1,4-dihydroxy-trans-nonene (DHN) and 4-hydroxy-trans-2-nonenoic acid (HNA), were modified (+30% DHN, -22% HNA) only at 3 MED, in parallel to the AR and ALDH enzyme activity modulation. In addition, a dose-dependent increase of unmodified HNE was found in the extracellular medium, paralleled by a significant fraction of the HNE-incubated dose not recovered at the intra- or extracellular level. The impairment of HNE metabolism paralleled a dramatic cytotoxic response. These results provide a reasonable explanation for the massive accumulation of carbonyl toxins in human skin in vivo after photoexposure and shed light on the detrimental effects of UVB radiation in the presence of unmetabolized LPO metabolites.

A simple and non-invasive visualization for assessment of carbonylated protein in the stratum corneum

BACKGROUND/PURPOSE

Stratum corneum (SC) is the interface of body and environment and is continuously exposed to oxidative stress, resulting in oxidative modification of proteins. Consequent carbonylated proteins (CPs) have so far been labeled with 2,4-dinitrophenyl (DNP) hydrazine and subsequently detected with anti-DNP antibody. We developed a simpler, non-invasive method to assess CP level in the SC and applied it to following research.

METHODS

SC was collected by adhesive tape stripping and its carbonyl groups were labeled with fluorescein-5-thiosemicarbazide (FTZ). The staining image was observed by fluorescence microscopy and the average fluorescence intensity of the SC extracted from the image was calculated as stratum corneum carbonylated protein (SCCP) level.

RESULTS

By reaction with FTZ, carbonyl groups in the SC could be detected easily. Relatively strong fluorescence was observed in exfoliating scales. Lipid removal from the SC in vitro or in vivo did not show any change in fluorescence intensity, suggesting that carbonyl groups were mainly derived from proteins, not from lipids. SCCP level was higher in the upper layer than the lower layer, and higher in the cheek (sun-exposed) than the inside of upper arm (unexposed), positively correlated with age especially in male cheek, positively correlated with transepidermal water loss, negatively correlated with water content, and showed a subtle correlation with sebum level. On the other hand, SC collected by cyanoacrylate resin and labeled with FTZ revealed strong fluorescence around the pores in the cheek and on the grooves in the upper arm, suggesting the role of sebum in the generation of SCCP.

CONCLUSION

SCCP was assessed in a simple and non-invasive method, and suggested to be a novel indicator that reflects some aspect of skin condition.

Protein oxidation and degradation during aging: role in skin aging and neurodegeneration

During aging, the products of oxidative processes accumulate and might disturb cellular metabolism. Among them are oxidized proteins and protein aggregates. On the other hand, in a functioning metabolic system oxidized proteins are degraded, mainly by the proteasome. During aging, however, proteasome activity declines. Therefore, the ability to degrade oxidized proteins is attenuated. The following review summarises the accumulation of oxidized proteins and the decline of the proteasomal system during skin and brain aging including some age-related neurodegenerative processes. The role of protein aggregates will be discussed as a potential reason for the accelerated dysfunction of tissue during aging.

Fructose 1,6-Diphosphate Alleviates UV-Induced Oxidative Skin Damage in Hairless Mice

Reactive oxygen species (ROS) are involved in the deleterious effects of UV light on skin. The antioxidant defense system is considered to be crucial for protecting skin from ROS. Recently, we showed that fructose 1,6-diphosphate (FDP), a glycolytic metabolite, reduced oxidative stress in UVB-irradiated keratinocytes. This study set out to determine whether topically applied FDP could exert protective effects against UV-induced skin damage in hairless mice. An in vitro skin permeation study using Franz-type diffusion cells showed that the amount of [14C]-FDP that diffused through the skin increased in a time-dependent manner, and about 3.5% of the applied FDP penetrated the skin after 24 h. Topical application of FDP (1%) preserved the endogenous antioxidant capacity of skin such as catalase and glutathione, which were significantly reduced after UVB irradiation without FDP. FDP also reversed the loss of catalase protein and prevented the accumulation of carbonylated proteins induced by UVB irradiation. These results provide evidence that topically administered FDP could penetrate into the skin and attenuate UVB-induced oxidative skin damage in hairless mice.

Generation and distribution of reactive oxygen species in the skin of hairless mice under UVA: studies on in vivo chemiluminescent detection and tape stripping methods

Although the formation of reactive oxygen species (ROS) in the skin induced by the ultraviolet (UV) light has been shown to lead to many cutaneous disorders, skin cancer and photoageing, the mechanism and distribution of ROS generation has not yet been definitively determined. In the present study, we examined the distribution of UVA-induced ROS in the skin of live hairless mice, using our proposed in vivo imaging chemiluminescent (CL) method to detect ROS combined with a CL probe (cypridina hilgendorfii luciferin analogue; CLA) and tape stripping (TS) technique. The CL intensities in the skin of live hairless mice were confirmed to significantly increase by UVA exposure. When TS was conducted five times in a maximum level after CL measurement following UVA exposure and subsequent CLA application, CL intensities due to UVA-induced ROS generation in the residual skin decreased to 10% of the original levels; and those in the stripped skin on each tape decreased in the stripped order such as 52%, 16%, 11%, 6% and 5%. Next, CLA was applied and then CL intensities were measured in the residual skin after advance 1, 3 and 5 tape strippings, and CL intensities due to ROS were detected primarily in the outer layer of the skin. On the basis of these results, we concluded that ROS induced by UVA exposure occurs and distributes in the outermost layer of the stratum corneum.

The repair enzyme peptide methionine-S-sulfoxide reductase is expressed in human epidermis and upregulated by UVA radiation

Recently, we reported that photoaging correlates well with the amount of oxidized protein accumulated in the upper dermis, while protein oxidation levels in the viable epidermis are very low. We hypothesized that this might be due to epidermal expression of the repair enzymes methionine sulfoxide reductases (MSRs). The expression of human methionine sulfoxide reductase A (MSRA) was investigated in HaCaT cells, primary human keratinocytes, and in human skin. High MSRA mRNA and protein levels as well as MSR activity were found in cultured human keratinocytes. MSRA was expressed in human epidermis, as shown by immunohistochemistry in healthy human skin. Repetitive in vivo exposure of human skin to solar-simulated light on 10 consecutive days (n=10 subjects) significantly increased epidermal MSRA expression. To further assess the functional relevance of the enzyme, its expression in response to UVB, UVA, and H(2)O(2) was investigated in HaCaT cells. While UVB lowered protein expression of MSRA, an upregulation was observed in response to low doses of UVA and H(2)O(2). In summary, MSRA represents the only enzyme so far identified in human skin that is capable of repairing oxidative protein damage. In addition to melanogenesis and DNA repair systems, a wavelength-specific activation of epidermal MSRA may be involved in epidermal photoprotection.

Functioning Methionine-S-Sulfoxide Reductases A and B Are Present in Human Skin

Methionine residues in the structure of proteins and peptides are especially sensitive to oxidation by hydrogen peroxide (H(2)O(2)) yielding both the (R) and (S) diastereomers of methionine sulfoxide. This commentary shows that both diastereomers of methionine sulfoxide (R and S) can be repaired in the human epidermis by methionine sulfoxide reductases A and B, respectively.

Functioning methionine sulfoxide reductases A and B are present in human epidermal melanocytes in the cytosol and in the nucleus

Oxidation of methionine residues by reactive oxygen (ROS) in protein structures leads to the formation of methionine sulfoxide which can consequently lead to a plethora of impaired functionality. The generation of methionine sulfoxide yields ultimately a diastereomeric mixture of the S and R sulfoxides. So far two distinct enzyme families have been identified. MSRA reduces methionine S-sulfoxide, while MSRB reduces the R-diastereomer. It has been shown that these enzymes are involved in regulation of protein function and in elimination of ROS via reversible methionine formation besides protein repair. Importantly, both enzymes require coupling to the NADPH/thioredoxin reductase/thioredoxin electron donor system. In this report, we show for the first time the expression and function of both sulfoxide reductases together with thioredoxin reductase in the cytosol as well as in the nucleus of epidermal melanocytes which are especially sensitive to ROS. Since this cell resides in the basal layer of the epidermis and its numbers and functions are reduced upon ageing and for instance also in depigmentation processes, we believe that this discovery adds an intricate repair mechanism to melanocyte homeostasis and survival.

UVA-induced modification of catalase charge properties in the epidermis is correlated with the skin phototype

The harmful effects of UVA radiation (320-400 nm) on the skin have been related to the generation of reactive oxygen species. Pheomelanin, the pigment characteristic of fair-skinned individuals, amplifies these effects. In vitro, in the presence of photosensitizing agents, UVA light produces singlet oxygen, which reacts with several targets. We have investigated a possible correlation between melanin-type and the antioxidant defense system after UV, focusing on the activities of superoxide dismutase and catalase, which correlated with the phototype of epidermal reconstructs. UVA was more effective than UVB in damaging these enzymatic activities, especially catalase. Furthermore, UVA irradiation induced a free-radical-mediated damage in the cells, leading to an oxidation of cell proteins. On catalase, synthetic pheomelanin amplified this effect on specific targets, such as residues of tryptophan and methionine. UVA irradiation of low phototype reconstructed epidermis and of U937 through synthetic pheomelanin induced a modification in the electrophoretic properties of native catalase, which was counteracted by histidine, a quencher of singlet oxygen. These results demonstrate that pheomelanin could act as a photosensitizing agent, following UVA irradiation, inducing charge modifications of native catalase, by a mechanism involving singlet oxygen or its downstream products.

Endogenous UVA-photosensitizers: mediators of skin photodamage and novel targets for skin photoprotection

Endogenous chromophores in human skin serve as photosensitizers involved in skin photocarcinogenesis and photoaging. Absorption of solar photons, particularly in the UVA region, induces the formation of photoexcited states of skin photosensitizers with subsequent generation of reactive oxygen species (ROS), organic free radicals and other toxic photoproducts that mediate skin photooxidative stress. The complexity of endogenous skin photosensitizers with regard to molecular structure, pathways of formation, mechanisms of action, and the diversity of relevant skin targets has hampered progress in this area of photobiology and most likely contributed to an underestimation of the importance of endogenous sensitizers in skin photodamage. Recently, UVA-fluorophores in extracellular matrix proteins formed posttranslationally as a consequence of enzymatic maturation or spontaneous chemical damage during chronological and actinic aging have been identified as an abundant source of light-driven ROS formation in skin upstream of photooxidative cellular stress. Importantly, sensitized skin cell photodamage by this bystander mechanism occurs after photoexcitation of sensitizers contained in skin structural proteins without direct cellular photon absorption thereby enhancing the potency and range of phototoxic UVA action in deeper layers of skin. The causative role of photoexcited states in skin photodamage suggests that direct molecular antagonism of photosensitization reactions using physical quenchers of photoexcited states offers a novel chemopreventive opportunity for skin photoprotection.

Carbonylation of cornified envelopes in the stratum corneum

Stratum corneum (SC), the outermost layer of the skin, is continuously exposed to oxidative stress via sunlight, lipid peroxidation, and is subsequently accompanied by oxidative modification. Previous studies have shown that major oxidative target proteins in the SC are keratins. However, it remains unclear to date whether cornified envelopes (CEs), protein envelopes of the corneocytes (cornified cells), would be oxidized. In this study, we first revealed oxidative modification of CEs using labeled hydrazide derivatives to detect carbonyl moieties. Carbonylation of CEs was confirmed by reaction with monoclonal antibodies against aldehyde-bound proteins, including anti-acrolein, anti-crotonaldehyde, anti-4-hydroxy-2-nonenal. The extent of carbonylation is stronger in CEs from the face, a sun-exposed area, than those from the inside of upper arm, an unexposed area. Carbonylation of CEs did not depend on their maturity, as evaluated by loss of involucrin antigenicity during maturation process, suggesting that CEs are carbonylated regardless of their maturation stage.

Interactions among stratum corneum defensive functions

With the exception of vitamin D production, virtually all epidermal functions can be considered as protective, or more specifically, as defensive in nature. Yet, the term "barrier function" of the stratum corneum (SC) is often used synonymously with only one such defensive function, although arguably its most important, i.e., permeability barrier homeostasis. Regardless of their hierarchy of relative importance, these critical protective functions largely reside in the SC. In this short review, we explore the ways in which the multiple defensive functions of the SC are linked and interrelated, either by their shared localization or by common biochemical processes.

Pravention und Regeneration epidermaler Barrierestorungen bei Berufsdermatosen. Prevention and regeneration of barrier disturbances in occupational dermatology

Over the past 10 years primary, secondary and tertiary prevention of occupational skin disorders has been shown to be successful, documented with appropriate statistical methods. Interventional strategies are the main features of secondary and tertiary prevention, now well-established in occupational dermatology. Primary prevention is best accomplished by health education measures, both in the form on individual counseling and seminars. This overview reviews the scientific background of hand eczema with respect to barrier damage and repair and then considers the options for individualized and focused prevention. Special anatomical features of the interdigital space and palms, as well as functional disorders, such as palmar hyperhidrosis, are discussed. The importance of barrier regeneration is considered in light of the role of an acid pH, the epidermal calcium gradient and aspects of percutaneous absorption. The effects of anti-oxidants are considered, and new bioengineering methods which rely on physiologic measuring techniques are reviewed.

[Cutaneous oxygen supply. With special consideration of skin uptake of oxygen from the atmosphere]

Oxygen reaches the skin from the blood as well as from the atmosphere. The diffusion gradient is a key factor in oxygen delivery. Measurements with puncture electrodes show a maximum oxygen partial pressure on the skin surface and a minimum in the deeper dermis. Non-invasive measures show that oxygen uptake depends on location. Assuming the oxygen consumption is 0.276+/-0.029 ml O(2).100 g(-1).min(-1) (37 degrees) and no oxygen is removed by capillaries, then the skin can be supplied by atmospheric oxygen to a depth of 0.25 to 0.40 mm, well into the dermis. Only the deeper dermis receives oxygen from the blood. The stratum corneum is a significant barrier to diffusion from the atmosphere. When this barrier is damaged, the oxygen diffusion increases Keratinocytes in culture proliferate in response to increased oxygen pressure; the same mechanism may explain epidermal hyperplasia following stratum corneum damage in vivo.

A chemiluminescence study of UVA-induced oxidative stress in human skin in vivo

Oxidative stress is defined as an imbalance between pro-oxidants and antioxidants in favor of pro-oxidants. Photon emission (also called chemiluminescence) has been widely used to study oxidative stress in biological systems in vitro. In vivo chemiluminescence has been proposed as a non-invasive method to assess oxidative stress in the skin. UVA (320-400 nm part of the ultraviolet radiation) exposure is generally accepted as a source of oxidative stress in the skin. In this study, UVA-induced oxidative stress was studied by using an in vivo chemiluminescence detection method. First, the dose response and the fluence rate response of the UVA-induced oxidative stress in human skin were investigated by examining the decay kinetics of the chemiluminescence signal following UVA exposure. A kinetic model was proposed to help differentiate these two responses. We found that the initial burst of the chemiluminescence signal depended on the UVA fluence rate, whereas the decay of the signal following exposure can be related to the UVA dose involved. Second, a significant reduction of UVA-induced chemiluminescence signal was observed after tape-stripping, indicating that stratum corneum is a major source of UVA-induced oxidative stress in the skin. Furthermore, the oxygen dependence of UVA-induced chemiluminescence signal was also confirmed by application of a pressure cuff, implying that some of the oxidative stress occurs in the deeper layers of the skin. Finally, topical application of vitamin C before exposure significantly reduced the UVA-induced chemiluminescence signal. We thus conclude that chemiluminescence is an effective method to assess the oxidative stress induced by UVA in human skin in vivo.

Keap1-null mutation leads to postnatal lethality due to constitutive Nrf2 activation

Transcription factor Nrf2 (encoded by Nfe2l2) regulates a battery of detoxifying and antioxidant genes, and Keap1 represses Nrf2 function. When we ablated Keap1, Keap1-deficient mice died postnatally, probably from malnutrition resulting from hyperkeratosis in the esophagus and forestomach. Nrf2 activity affects the expression levels of several squamous epithelial genes. Biochemical data show that, without Keap1, Nrf2 constitutively accumulates in the nucleus to stimulate transcription of cytoprotective genes. Breeding to Nrf2-deficient mice reversed the phenotypic Keap1 deficiencies. These experiments show that Keap1 acts upstream of Nrf2 in the cellular response to oxidative and xenobiotic stress.

Ultraviolet a induces generation of squalene monohydroperoxide isomers in human sebum and skin surface lipids in vitro and in vivo

At the outermost surface of human skin, skin surface lipids are first-line targets of solar ultraviolet radiation. Therefore, we hypothesized that ultraviolet A and ultraviolet B irradiation induce photo-oxidation of skin surface lipids. To test this, sebum samples were collected from facial skin of 17 healthy volunteers, weighed, and immediately irradiated with either ultraviolet B or ultraviolet A. Squalene, the major sebum lipid, as well as photo-oxidation products were identified in sebum lipid extracts by high-performance liquid chromatography analysis. Upon ultraviolet A exposures squalene was depleted in a concentration-dependent manner, whereas an unidentified sebum lipid photo-oxidation product was detected. Using high-performance thin layer chromatography, high-performance liquid chromatography, atmospheric pressure chemical ionization mass spectrometry, and nuclear magnetic resonance, unidentified sebum lipid photo-oxidation product was identified as a mixture of squalene monohydroperoxide isomers. Squalene monohydroperoxide isomers purified from sebum were identical with squalene monohydroperoxide isomers synthesized by preparative photo-oxidation of squalene. Squalene monohydroperoxide isomers were formed even after small suberythematogenic doses of ultraviolet A (5 J per cm2). Whereas physiologic baseline levels of squalene monohydroperoxide isomers in human skin were only slightly above detection limits, squalene monohydroperoxide isomer levels were strongly increased by suberythematogenic doses of ultraviolet A both in vitro and in vivo. High-performance liquid chromatography results could be complemented by a straightforward thin layer chromatography method for rapid screening of lipid peroxide formation in human sebum/skin surface lipids. In conclusion, specific squalene monohydroperoxide isomers were identified as highly ultraviolet A sensitive skin surface lipid breakdown products that may serve as a marker for photo-oxidative stress in vitro and in vivo.

The role of H2O2 as a mediator of UVB-induced apoptosis in keratinocytes

Apoptosis is an active form of cell death that is initiated by a variety of stimuli, including reactive oxygen species (ROS) and ultraviolet (UV) radiation. Previously, it has been reported that UVB-irradiation of keratinocytes leads to intracellular generation of hydrogen peroxide (H2O2) and that antioxidants can inhibit ROS-induced apoptosis. Although both UVB-irradiation and H2O2-incubation led to increased intracellular H2O2 levels, the antioxidants catalase and glutathione monoester (GME), inhibited apoptosis only when induced by H2O2, not by UVB. Furthermore, extracellular signal-regulated kinase (ERK), a prominent member of the mitogen-activated protein kinase (MAPK) family, was found to be activated by treatment with both UVB and H2O2. Inhibition of ERK phosphorylation by pre-treatment with PD98059 resulted in enhanced apoptosis after H2O2-exposure. However,no significant difference of apoptosis was observed between cells with and without inhibitor pre-treatment upon UVB-irradiation. DNA damage in the form of cyclobutane pyrimidine dimers was observed after exposure to UVB, but no photoproducts were found in H2O2-treated cells. These results suggest a ROS-independent pathway of UVB-induced apoptosis. Although UVB-irradiation causes moderate increase in H2O2, the generation of H2O2 does not contribute to the induction of apoptosis. Moreover, activation of ERK only blocks H2O2-dependent apoptosis but has no impact on UVB-induced apoptosis.

Antioxidant enzyme activity in human stratum corneum shows seasonal variation with an age-dependent recovery

The stratum corneum, as the body's principal barrier to the environment, is continuously exposed to environmental sources of reactive oxygen species like ultraviolet light, ozone, and pollution. Reactive oxygen species are believed to be involved in cancer, aging, and inflammatory skin disorders. We have developed a method to measure catalase and superoxide dismutase activity on tape strippings from the human stratum corneum and demonstrated a gradient of antioxidant enzyme activity across the stratum corneum with decreasing levels towards the skin surface. Sun exposure resulted in a seasonal variation of the catalase activity in stratum corneum, with low activities in summer and higher activities in winter for the same person, whereas superoxide dismutase activity in stratum corneum did not seem to vary in those conditions. Exposure of human skin to broadband ultraviolet-A resulted in a dose-dependent deactivation of the catalase activity in stratum corneum within 24 h, whereas exposure to ultraviolet-B had no effect. Superoxide dismutase activity in stratum corneum was not affected by ultraviolet-A or ultraviolet-B irradiation within 24 h. After exposure to a dose of 15 J per cm2 broadband ultraviolet-A, full recovery of the catalase activity occurred in 3-4 wk at an age-dependent rate. We conclude that sun exposure results in a disturbed catalase to superoxide dismutase ratio in the stratum corneum. This may lead to an increased vulnerability to oxidative damage in stratum corneum barrier components. These results therefore stress the importance of providing efficient protection for this internal defense mechanism in sun-exposed areas of the skin.

Topical alpha-tocotrienol supplementation inhibits lipid peroxidation but fails to mitigate increased transepidermal water loss after benzoyl peroxide treatment of human skin

Benzoyl peroxide (BPO) is a commonly used drug in the treatment of acne vulgaris, but it induces unwanted side effects related to stratum corneum (SC) function. Since it has been recently shown to oxidize SC antioxidants, it was hypothesized that antioxidant supplementation may mitigate the BPO-induced SC changes. To test this, 11 subjects were selected to be topically supplemented with alpha-tocotrienol (5% w/vol) for 7 d on defined regions of the upper back, while the contralateral region was used for vehicle-only controls. Starting on day 8, all test sites were also treated with BPO (10%) for 7 d; the alpha-tocotrienol supplementation was continued throughout the study. A single dose of BPO depleted 93.2% of the total vitamin E. While continuing the BPO exposure for 7 d further depleted vitamin E in both vehicle-only and alpha-tocotrienol-treated sites, significantly more vitamin E remained in the alpha-tocotrienol-treated areas. Seven BPO applications increased lipid peroxidation. Alpha-tocotrienol supplementation significantly mitigated the BPO-induced lipid peroxidation. The transepidermal water loss was increased 1.9-fold by seven BPO applications, while there was no difference between alpha-tocotrienol treatment and controls. The data suggest that alpha-tocotrienol supplementation counteracts the lipid peroxidation but not the barrier perturbation in the SC induced by 10% BPO.

What is the role of alpha-linolenic acid for mammals?

This review examines the data pertaining to an important and often underrated EFA, alpha-linolenic acid (ALA). It examines its sources, metabolism, and biological effects in various population studies, in vitro, animal, and human intervention studies. The main role of ALA was assumed to be as a precursor to the longer-chain n-3 PUFA, EPA and DHA, and particularly for supplying DHA for neural tissue. This paper reveals that the major metabolic route of ALA metabolism is beta-oxidation. Furthermore, ALA accumulates in specific sites in the body of mammals (carcass, adipose, and skin), and only a small proportion of the fed ALA is converted to DHA. There is some evidence that ALA may be involved with skin and fur function. There is continuing debate regarding whether ALA has actions of its own in relation to the cardiovascular system and neural function. Cardiovascular disease and cancer are two of the major burdens of disease in the 21st century, and emerging evidence suggests that diets containing ALA are associated with reductions in total deaths and sudden cardiac death. There may be aspects of the action and, more importantly, the metabolism of ALA that need to be elucidated, and these will help us understand the biological effects of this compound better. Additionally, we must not forget that ALA is part of the whole diet and should be seen in this context, not in isolation.

Role of ascorbic acid in stratum corneum lipid models exposed to UV irradiation

PURPOSE

The effects of ascorbic acid on Stratum corneum lipid models following ultraviolet irradiation were studied adding iron ions as transition metal catalysts.

METHODS

Lipid peroxidation was quantified by the thiobarbituric acid assay. The qualitative changes were studied on a molecular level by mass spectrometry. To elucidate the nature of free radical involvement we carried out electron paramagnetic resonance studies. The influence of ascorbic acid on the concentration of hydroxyl radicals was examined using the spin trapping technique. Moreover, we checked the vitamin's ability to react with stable radicals.

RESULTS

Ascorbic acid was found to have prooxidative effects in all lipid systems in a concentration dependent manner. The degradation products of ascorbic acid after its prooxidative action were detected. The concentration of the hydroxyl radicals in the Fenton assay was decreased by ascorbic acid. The quantification assay of 2,2-diphenyl-1-picrylhydrazyl hydrate showed reduced concentration levels of the stable radical caused by ascorbic acid.

CONCLUSIONS

Considering human skin and its constant exposure to UV light and oxygen, an increased pool of iron ions in irradiated skin and the depletion of co-antioxidants, the administration of ascorbic acid in cosmetic formulations or in sunscreens could unfold adverse effects among the Stratum corneum lipids.

Photoaging is associated with protein oxidation in human skin in vivo

There is increasing evidence for the generation of reactive oxygen species in skin upon ultraviolet exposure, but little is known about their pathophysiologic relevance in human skin in vivo. We hypothesized that chronic and acute photodamage is mediated by depleted antioxidant enzyme expression and increased oxidative protein modifications. Biopsies from patients with histologically confirmed solar elastosis, from non-ultraviolet-exposed sites of age-matched controls, and from young subjects were analyzed. To evaluate the influence of acute ultraviolet exposures, buttock skin of 12 healthy subjects was irradiated repetitively on 10 d with a solar simulator and compared intraindividually to non-ultraviolet-treated contralateral sites. The antioxidant enzymes catalase, copper-zinc superoxide dismutase, and manganese superoxide dismutase were investigated by immunohistochemistry. Protein carbonyls were analyzed by immunohistochemical and immunoblotting techniques in human skin and in cell models. Whereas overall expression of antioxidant enzymes was very high in the epidermis, low baseline levels were found in the dermis. In photoaged skin, a significant depletion of antioxidant enzyme expression was observed within the stratum corneum and in the epidermis. Importantly, an accumulation of oxidatively modified proteins was found specifically within the upper dermis of photoaged skin. Upon acute ultraviolet exposure of healthy subjects, depleted catalase expression and increased protein oxidation were detected. Exposures of keratinocytes and fibroblasts to ultraviolet B, ultraviolet A, and H2O2 led to dose-dependent protein oxidation and thus confirmed in vivo results. In conclusion, the correlation between photodamage and protein oxidation was demonstrated for the first time, which hence may be a relevant pathophysiologic factor in photoaging.

Identification of alpha-dicarbonyl scavengers for cellular protection against carbonyl stress

Tissue deterioration and aging have long been associated with the accumulation of chemically induced protein and DNA damage. Reactive oxygen species (ROS) and reactive carbonyl species (RCS), especially alpha-dicarbonyl compounds, are key mediators of damage caused by oxidative stress, glycation, and UV-irradiation. The toxic effects of ROS are counteracted in vivo by antioxidants and antioxidant enzymes, and the deleterious effects of one RCS, methylglyoxal, are counteracted by a ubiquitous glyoxalase system. Carbonyl stress as a result of toxic effects of various mono-dicarbonyls (e.g. 4-hydroxynonenal) and alpha-dicarbonyls (e.g. glyoxal and deoxyosones) cannot be directly antagonized by antioxidants, and only a small number of biological carbonyl scavengers like glutathione (GSH) have been identified to date. We have developed a new screening method for the identification of carbonyl scavengers using a rapid glycation system that proceeds independent of oxygen and therefore, excludes identification of inhibitory compounds acting as antioxidants. Using this screening assay adapted to 96-well microtiter plates, we have identified the cysteine derivative 3,3-dimethyl-D-cysteine as a potent inhibitor of non-oxidative advanced glycation. Comparative kinetic analyses demonstrated the superior alpha-oxoaldehyde-scavenging activity of D-penicillamine over that of aminoguanidine. D-Penicillamine traps alpha-oxoaldehydes by forming a 2-acylthiazolidine derivative as shown by structure elucidation of reaction products between D-penicillamine and methylglyoxal or phenylglyoxal. We demonstrated that upon co-incubation, D-penicillamine protects human skin keratinocytes and fibroblasts (CF3 cells) against glyoxal- and methylglyoxal-induced carbonyl toxicity. Our research qualifies alpha-amino-beta-mercapto-beta,beta-dimethyl-ethane as a promising pharmacophore for the development of related alpha-dicarbonyl scavengers as therapeutic agents to protect cells against carbonyl stress.

Environmental effects on the functions of the stratum corneum

The stratum corneum (SC) is such an efficient barrier that only 2-5 g per h per cm2 of transepidermal water loss (TEWL) occurs in normal skin. The SC also plays another important role at the skin surface in keeping our skin smooth and flexible by binding water. We exposed a simulation model of in vivo SC to various, excessive physical insults in vitro, such as irradiation with 1 J per cm2 of UVB, 50 J per cm2 of UVA, or 3000 rad of X-ray, heating at 90 degrees C for 3 min, freezing at -196 degrees C for 60 s or repeated placement in an extremely dry or humid condition. None of them could cause any permanent change in the SC functions. Only the application of chemical agents such as lipid solvents or a detergent or the affliction of trauma resulted in a functional derangement of the SC. Because the viable skin tissues are more vulnerable to the effects of the environment than the SC, most of the abnormalities of the SC functions developing after environmental insults are secondarily caused by enhanced epidermal proliferation induced under the influence of underlying inflammation. These functional abnormalities were found to be demonstrable with biophysical measurements long after the disappearance of skin redness, the clinically observable sign of inflammation. The SC abnormalities in inflamed skin are also detectable as a change in the content of chemical mediators. For example, the ratio between proinflammatory IL-1 and its receptor antagonist (IL-1ra) whose production by epidermal keratinocytes is markedly enhanced by various proinflammatory stimuli, showed a deviation towards an excess of the latter in inflammatory skin. Facial skin that is always exposed to the environment is unique in that its SC shows such a deviation in the IL-1/IL-1ra ratio suggestive for the presence of mild inflammation even in normal individuals.

The proteasome and its function in the ageing process

The ageing process is characterized by a progressive loss of function and a decline in the functional capacities of the organism, leading to death. The nature of the processes involved in loss of functions is not well understood. A number of theories have been proposed, including a hypothesis that emphasizes the role of reactive oxygen species as a fundamental causal factor in the ageing process; among other things, oxidative damage to proteins through reactive oxygen species plays a key role in the ageing process. Oxidative modification of proteins generally causes them to become dysfunctional, and normally to undergo preferential degradation. Within the cell the main proteolytic machinery involved in the degradation of oxidized proteins is the proteasomal system, consisting of a multicatalytic protease complex--the proteasome--and numerous regulatory factors. The proteasome is a highly conserved structure that is distributed in the cytosol, nucleus and endoplasmatic reticulum of mammalian cells. As the proteasome itself is also exposed to oxidative stress during the ageing process several studies were carried out to investigate the role and the activity of the proteasomal system during ageing. This review will describe current knowledge of the activity of the protesomal system and its possible involvement in the ageing process.

Hydrogen peroxide induced stress in human keratinocytes and its effect on bithionol toxicity

Exposure to hydrogen peroxide causes oxidative stress in keratinocytes. Previous work has shown that the antiparasitic drug bithionol has an EC(50) of 0.7 microg/ml (2 microM) with primary human keratinocytes, but that these cells do not respond to photoactivated bithionol. Bithionol is known to be photoactivated by UV-A visible light, therefore this study aims to investigate the effects of inducing oxidative stress in the cells prior to bithionol treatment alone and in the presence of UV-A visible light. Oxidative stress, by hydrogen peroxide treatment, caused the cells to become sensitive to photoactivated bithionol. Bithionol alone reduced the amount of oxidative stress, while following photoactivation, an augmentation in the amount of oxidative stress and cell cytotoxicity was observed. The hydrogen peroxide treatment did not alter the sensitivity of the keratinocytes to 5 J/cm(2) UV-A visible light.

Vitamin C, uric acid, and glutathione gradients in murine stratum corneum and their susceptibility to ozone exposure

The stratum corneum has been recognized as the main cutaneous oxidation target of atmospheric ozone (O3), a major part of photochemical smog. This study reports the presence and distribution of vitamin C, glutathione, and uric acid in murine stratum corneum, and evaluates their susceptibility to acute environmental exposure to O3. Based on tape stripping and a modified extraction method with high performance liquid chromatography electrochemical analysis, we detected vitamin C (208.0 +/- 82.5 pmol per 10 consecutive pooled tapes), glutathione (283.7 +/-96.3), and uric acid (286.4 +/-47.1) in murine stratum corneum as compared with only 16.5 +/- 1.4 pmol alpha-tocopherol. Vitamin C, glutathione (both p < 00.001), and urate (p < 0.01) were found to exhibit a gradient with the lowest concentrations in the outer layers and a steep increase in the deeper layers. To investigate the effect of O3 exposure on hydrophilic antioxidants, we exposed SKH-1 hairless mice to O3 concentrations of 0, 0.8, 1, and 10 p.p.m., and stratum corneum was analyzed before and after exposure. Whereas mock exposure with 0 p.p. m. for 2 h had no significant effect, O3 doses of 1 p.p.m. for 2 h and above showed depletion of all three antioxidants. Vitamin C was decreased to 80% +/- 15% of its pretreatment content (p < 0.05), GSH to 41% +/- 24% (p < 0.01), and uric acid to 44% +/- 28% (p < 0.01). This report demonstrates the previously unrecognized role of hydrophilic antioxidants in the stratum corneum and provides further evidence that O3 induces oxidative stress in this outer skin layer.