Comedogenicity of squalene monohydroperoxide in the skin after topical application

Induction of ferroptosis in human keratinocyte HaCaT cells by squalene hydroperoxide: Possible prevention of skin ferroptosis by botanical extracts

Ever since the proposal of ferroptosis, it has been studied as a nonapoptotic cell death caused by iron ion-dependent phospholipid (PL) peroxidation. We previously showed that treatment of human hepatoma cell line HepG2 with prepared PL hydroperoxide (PLOOH) resulted in ferroptosis. However, in human sebum, the major hydroperoxide is not PLOOH but squalene hydroperoxide (SQOOH), and to our knowledge, it is not established yet whether SQOOH induces ferroptosis in the skin. In this study, we synthesized SQOOH and treated human keratinocyte HaCaT cells with SQOOH. The results showed that SQOOH induces ferroptosis in HaCaT cells in the same way that PLOOH causes ferroptosis in HepG2 cells. Some natural antioxidants (botanical extracts) could inhibit the ferroptosis in both the cell types. Consequently, future research focus would revolve around the involvement of SQOOH-induced ferroptosis in skin pathologies as well as the prevention and treatment of skin diseases through inhibition of ferroptosis by botanical extracts.

Squalene Peroxidation and Biophysical Parameters in Acne-Prone Skin: A Pilot "In Vivo" Study

Nowadays, acne vulgaris therapies are often unsuccessful. One of the responsible factors for the formation of comedones and inflammatory lesions could be the peroxidation of squalene, a hydrocarbon representing one of the major components of human sebum. This peroxidation is increased by solar irradiation. The purpose of this work was to set up an in vivo method for the extraction and quantification of squalene from acne skin and to correlate the results with biophysical skin parameters such as sebum amount, protein content and TEWL. Healthy volunteers were used as control. The results obtained demonstrated that acne-prone skin had a major quantity of squalene, and, in the stratum corneum area, its peroxide form is present. Moreover, Spearman's rank correlation showed a positive correlation between sebum content and peroxide squalene and between porphyrin intensity and peroxide squalene.

Squalane and Squalene

The Expert Panel for Cosmetic Ingredient Safety reviewed newly available studies since their original assessment in 1982, along with updated information regarding product types and concentrations of use, and confirmed that Squalane and Squalene are safe as cosmetic ingredients in the practices of use and concentration as described in this report.

Significance of Singlet Oxygen Molecule in Pathologies

Reactive oxygen species, including singlet oxygen, play an important role in the onset and progression of disease, as well as in aging. Singlet oxygen can be formed non-enzymatically by chemical, photochemical, and electron transfer reactions, or as a byproduct of endogenous enzymatic reactions in phagocytosis during inflammation. The imbalance of antioxidant enzymes and antioxidant networks with the generation of singlet oxygen increases oxidative stress, resulting in the undesirable oxidation and modification of biomolecules, such as proteins, DNA, and lipids. This review describes the molecular mechanisms of singlet oxygen production in vivo and methods for the evaluation of damage induced by singlet oxygen. The involvement of singlet oxygen in the pathogenesis of skin and eye diseases is also discussed from the biomolecular perspective. We also present our findings on lipid oxidation products derived from singlet oxygen-mediated oxidation in glaucoma, early diabetes patients, and a mouse model of bronchial asthma. Even in these diseases, oxidation products due to singlet oxygen have not been measured clinically. This review discusses their potential as biomarkers for diagnosis. Recent developments in singlet oxygen scavengers such as carotenoids, which can be utilized to prevent the onset and progression of disease, are also described.

Influence of Mechanical Ventilation Systems and Human Occupancy on Time-Resolved Source Rates of Volatile Skin Oil Ozonolysis Products in a LEED-Certified Office Building

Building mechanical ventilation systems are a major driver of indoor air chemistry as their design and operation influences indoor ozone (O₃) concentrations, the dilution and transport of indoor-generated volatile organic compounds (VOCs), and indoor environmental conditions. Real-time VOC and O₃ measurements were integrated with a building sensing platform to evaluate the influence of mechanical ventilation modes and human occupancy on the dynamics of skin oil ozonolysis products (SOOPs) in an office in a LEED-certified building during the winter. The ventilation system operated under variable recirculation ratios (RRs) from RR = 0 (100% outdoor air) to RR = 1 (100% recirculation air). Time-resolved source rates for 6-methyl-5-hepten-2-one (6-MHO), 4-oxopentanal (4-OPA), and decanal were highly dynamic and changed throughout the day with RR and occupancy. Total SOOP source rates during high-occupancy periods (10:00-18:00) varied from 2500-3000 μg h^-1 when RR = 0.1 to 6300-6700 μg h^-1 when RR = 1. Source rates for gas-phase reactions, outdoor air, and occupant-associated emissions generally decreased with increasing RR. The recirculation air source rate increased with RR and typically became the dominant source for RR > 0.5. SOOP emissions from surface reservoirs were also a prominent source, contributing 10-50% to total source rates. Elevated per person SOOP emission factors were observed, potentially due to multiple layers of soiled clothing worn during winter.

Novel Photoinduced Squalene Cyclic Peroxide Identified, Detected, and Quantified in Human Skin Surface Lipids

Skin surface lipids (SSLs) form the first barrier that protects the human organism from external stressors, disruption of the homeostasis of SSLs can result in severe skin abnormalities. One of the main causes of this disruption is oxidative stress that is primarily due to SSLs oxidation. Squalene (SQ), the most abundant lipid among SSLs, was shown to first undergo singlet molecular oxygen (1O2) oxidation to yield 6 SQ-monohydroperoxide (SQ-OOH) isomers as the primary oxidation products. However, due to the instability and lability of hydroperoxides, we found that when total SQ-OOH isomers are further photooxidized, they form a unique higher molecular weight secondary oxidation product. To generate the compound, we photooxidized total SQ-OOH isomers in the presence of ground state molecular oxygen (3O2), after its isolation and purification, we studied its structure using MS/MS, NMR, derivatization reactions, and chemical calculations. The compound was identified as 2-OOH-3-(1,2-dioxane)-SQ. Photooxidation of individual SQ-OOH isomers revealed that 6-OOH-SQ is the precursor of 2-OOH-3-(1,2-dioxane)-SQ and indicated the possibility of the formation of similar cyclic peroxides from each isomer following the same photoinduced chain reaction mechanism. An HPLC-MS/MS method was developed for the analysis of 2-OOH-3-(1,2-dioxane)-SQ and its presence on the skin was confirmed in SSLs of six healthy individuals. Its quantity on the skin correlated directly to that of SQ and was not inversely proportional to its precursor, indicating the possibility of its accumulation on the skin surface and the constant regeneration of 6-OOH-SQ from SQ's oxidation. In general, research on lipid cyclic peroxides in the human organism is very limited, and especially on the skin. This study shows for the first time the identification and presence of a novel SQ cyclic peroxide "2-OOH-3-(1,2-dioxane)-SQ" in SSLs, shedding light on the importance of further studying its effect and role on the skin.

Lipidomics of facial sebum in the comparison between acne and non-acne adolescents with dark skin

Lipidomics is advantageous in the study of sebum perturbations occurring in acne. An extended evaluation of the sebum lipid profiles in acne-prone sebaceous areas is lacking in dark skin. Yet, there is a void space in understanding how the building blocks of sebum lipids, i.e. individual fatty acids (FAs), are intertwined with acne-prone skin. We aimed to determine the sebum lipidome in facial areas of adolescents with and without acne in Nigeria. A cross-sectional analytical study was conducted in 60 adolescents/young adults divided in 30 acne patients (15F, 15M) and 30 age and sex-matched controls. Sebum samples obtained from foreheads and cheeks were analysed separately by gas chromatography–mass spectrometry (GCMS) and thin layer chromatography (HPTLC). Distributions of sebum components were investigated with multivariate ANOVA-simultaneous component analysis (ASCA). Sebum incretion in acne was paralleled by significantly higher abundance of triglycerides, wax esters, and squalene together with monounsaturated FAs (MUFAs), and straight chain saturated FAs (SFAs), especially those with odd-carbon chain, i.e. C13:0, C15:0, and C17:0. Profiling weight/weight percentage of individual components revealed that, in acne, the free FAs (FFAs) array was shifted towards higher relative abundance of the SFAs C15:0, C16:0, and C17:0 and lower percentage of the anteiso-branched FFAs with 12, 14, 16, and 18 carbons. In acne patients, MUFAs and PUFAs were quantitatively increased and decreased on foreheads and cheeks, respectively. Relative abundance of fatty alcohols was decreased in acne independent on the site. The results indicated that acne associates with site-specific derangement of the pathways regulating the balance among odd straight-chain and branched-chain SFAs, MUFAs, which included sapienate (C16:1n-10), PUFAs, and squalene.

Linoleic acid and squalene are oxidized by discrete oxidation mechanisms in human sebum

Previous studies suggest that squalene (SQ) in sebum is oxidized by a photooxidation mechanism (i.e., singlet oxygen oxidation) to create SQ hydroperoxide (SQOOH), a compound that causes adverse skin conditions. However, oxidation of other lipids in sebum, such as linoleic acid (LA), has not been fully understood. Elucidating their oxidation, especially its mechanisms, may lead to a further understanding of the relationship between sebum oxidation and skin conditions. In this study, using HPLC-MS/MS, we aimed to detect LA hydroperoxide (LAOOH) directly from sebum and identify the oxidation mechanism of LA in sebum through analysis of LAOOH isomers. We developed extraction and HPLC-MS/MS analysis conditions that can sufficiently quantify each LAOOH isomer in sebum. Using this method, LAOOH was detected in samples from healthy individuals, demonstrating the presence of LAOOH in human sebum. Moreover, isomer analysis of LAOOH and SQOOH indicated that LA and SQ are oxidized in sebum by discrete oxidation mechanisms (LA oxidized by free radical oxidation, whereas SQ oxidized by singlet oxygen oxidation). Such results may further lead to the development of mechanism-specific ways to prevent oxidation of sebum via a selection of appropriate antioxidants, ultimately leading to the promotion of skin health.

Skin lipids in health and disease: A review

Our skin is the interface between us and our environment - a flexible barrier that has evolved for protection, immunity, regulation and sensation. Once regarded as inert, we now know that it is a dynamic environment. Skin lipids are crucial to the structure and function of skin. From deep in the hypodermis, through the ceramide-rich epidermis, to the lipids of the skin surface, there are a vast array of different lipids with important roles to play. This review firstly discusses the lipid composition of human skin and secondly, changes that have been found in skin lipid composition in different skin diseases. Further research into skin lipids facilitated by ever-improving methodologies will no doubt generate new knowledge, paving the way for diagnosis, prevention and treatment of skin disorders and diseases.

Yields and Variability of Ozone Reaction Products from Human Skin

The skin of 20 human participants was exposed to ∼110 ppb O₃ and volatile products of the resulting chemistry were quantified in real time. Yields (ppb product emitted/ppb ozone consumed) for 40 products were quantified. Major products of the primary reaction of ozone-squalene included 6-methyl 5-hepten-2-one (6-MHO) and geranyl acetone (GA) with average yields of 0.22 and 0.16, respectively. Other major products included decanal, methacrolein (or methyl vinyl ketone), nonanal, and butanal. Yields varied widely among participants; summed yields ranged from 0.33 to 0.93. The dynamic increase in emission rates during ozone exposure also varied among participants, possibly indicative of differences in the thickness of the skin lipid layer. Factor analysis indicates that much of the variability among participants is due to factors associated with the relative abundance of (1) "fresh" skin lipid constituents (such as squalene and fatty acids), (2) oxidized skin lipids, and (3) exogenous compounds. This last factor appears to be associated with the presence of oleic and linoleic acids and could be accounted for by uptake of cooking oils or personal care products to skin lipids.

Lipidomics reveals the role of glycoceramide and phosphatidylethanolamine in infantile acne

BACKGROUND

Infantile acne refers to a skin disease that usually occurs on the face from 6 weeks to 1 year old, and the etiology of infantile acne is different from that of puberty acne. The change of skin surface lipid (SSL) is an important factor in the occurrence of acne.

OBJECTIVES

Comparison of facial skin lipid changes in healthy and acne-prone infants, screening for possible groupings of substances, to explore factors associated with the development of acne in infants, such as gender and frequency of face washing.

METHODS

A questionnaire was used to obtain information about the infant, facial lipids were affixed with adhesive tape, and the samples were analyzed using Ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS), with subsequent data analyzed by single-factor and multifactor analysis.

RESULTS

It was observed that some lipids followed the trend of lipid master classes, subclasses and were significantly different. Galactosylceramides, glycosylceramides and phosphatidylethanolamine were screened as the majority of differentiating lipid species. Additionally, it was emphasized that the frequency of face washing and parent's acne was independent with the severity of acne in infant by clinical characteristics, whereas the skin type and gender can affect the severity of acne statistically.

CONCLUSIONS

Facial lipids change significantly between infantile acne and healthy infants, suggesting that qualitative and quantitative changes in lipids are strongly associated with the development of acne and may exacerbate the environment in which acne occurs.

Squalene Oxidation Induced by Urban Pollutants: Impact on Skin Surface Physico-Chemistry

The effect of urban pollutants on skin properties has been revealed through several epidemiological studies. However, comprehension of involved mechanisms remains undetermined. In addition, the impact of such stressors on skin surface properties, especially skin physico-chemistry, has not been investigated. Consequently, the present study aims to develop a new aging protocol able to highlight the impact of selected urban pollutants on a model sebaceous lipid: the squalene. Its quality has been followed during aging using LC-MS analysis. Results showed that the quality of the control solution containing only squalene remains stable during 45 days, whereas the quality of the solution containing squalene mixed with pollutants appears greatly altered, especially in the presence of heavy metals: a large amount of oxidation compounds was evidenced due to oxidation and dehydrogenation mechanisms. In addition, a physicochemical study was performed using a validated nonbiological skin model able to mimic skin physico-chemistry. Surface free energy components were calculated using contact angle measurements according to the Van Oss model. The application of degraded squalene significantly increased skin hydrophilic and monopolar behavior compared to the application of control squalene. Those modifications are essentially explained by the nature of squalene oxidation products. It must be noted from this study that squalene oxidation due to pollutants or due to high temperature did not lead to the same physicochemical consequences neither to the same oxidation products, as shown by thermal analysis. This study gives original and precious information to explain alterations induced by pollutants on skin surface properties, especially skin chemistry and physico-chemistry.

Oxidation of squalene by singlet oxygen and free radicals results in different compositions of squalene monohydroperoxide isomers

Oxidation of squalene (SQ) causes a decline in the nutritional value of SQ in foods, as well as an accumulation of SQ oxidation products in skin lipids which lead to adverse skin conditions. However, mechanistic insights as to how SQ is oxidized by different oxidation mechanisms have been limited, and thus effective measures towards the prevention of SQ oxidation have not been identified. In this study, we oxidized SQ by either singlet oxygen oxidation or free radical oxidation, and monitored the formation of the six SQ monohydroperoxide (SQOOH) isomers, the primary oxidation products of SQ, at the isomeric level. While singlet oxygen oxidation of SQ resulted in the formation of similar amounts of the six SQOOH isomers, free radical oxidation of SQ mainly formed two types of isomers, 2-OOH-SQ and 3-OOH-SQ. The addition of β-carotene during singlet oxygen oxidation, and the addition of α-tocopherol during free radical oxidation lead to a dose-dependent decrease in the formation of SQOOH isomers. Such results suggest that the analysis of SQOOH at the isomeric level allows for the determination of the cause of SQ oxidation in various samples, and provides a foothold for future studies concerning the prevention of SQ oxidation.

Sebaceous-immunobiology is orchestrated by sebum lipids

The major role of sebaceous glands in mammals is to produce sebum, which coats the epidermis and the hair providing waterproofing, thermoregulation and photoprotection. However, as the need for these functions decreased along the evolutionary changes in humans, a relevant question has been raised: are sebaceous glands and sebum the remnants of our mammalian heritage or do they have overtaken a far more complex role in human skin biology? Trying to provide answers to this question, this review introduces the evolving field of sebaceous immunobiology and puts into the focus the pathways that sebum lipids use to influence the immune milieu of the skin. By introducing possible modifiers of sebaceous lipogenesis and discussing the – human-specific – alterations in composition and amount of sebum, the attribute of sebum as a sensitive tool, which is capable of translating multiple signalling pathways into the dermal micro environment is presented. Further their interaction with macrophages and keratinocytes involves sebum lipid fractions into disease pathogenesis, which could lead – on the other side – to the development of novel sebum-based therapeutic strategies.

Assessment of a new biological complex efficacy on dysseborrhea, inflammation, and Propionibacterium acnes proliferation

INTRODUCTION

Acne vulgaris is a common chronic inflammatory disease of the pilosebaceous unit triggered by Propionibacterium acnes. A bakuchiol, Ginkgo biloba extract, and mannitol (BGM) complex has been developed to provide patients with acne with a specific dermocosmetic to be used adjunctively with conventional treatments.

OBJECTIVE

The aim of these studies was to assess the antibacterial, anti-inflammatory, and antioxidative potential of BGM complex and its individual compounds as well as its impact on sebum composition.

METHODS

The antibacterial, anti-inflammatory, and antioxidative potential of BGM complex and its compounds was assessed through in vitro, ex vivo, and clinical studies. The clinical benefit of BGM complex formulated in a cream was assessed in subjects prone to acne through sebum composition analysis and photometric assessments.

RESULTS

Results from the studies showed that the BGM complex has significant antibacterial, anti-inflammatory, and antioxidative properties. At similar concentrations, bakuchiol has up to twice the antioxidative potential than vitamin E. In subjects, BGM complex regulated the sebum composition in acne patients by increasing the level of sapienic and linolenic acid and reducing the level of oleic acid. The reduced number of porphyrins on the skin surface showed that it is also effective against P. acnes.

CONCLUSION

BGM complex provides a complete adjunctive care in patients with acne by targeting etiopathogenic factors of acne: dysseborrhea, inflammation, and P. acnes proliferation.

Analysis of comedone, sebum and porphyrin on the face and body for comedogenicity assay

BACKGROUND/PURPOSE

Many ingredients used in cosmetics evoke a comedogenic response. Rabbit ear model (REM) is a useful method that can replace human in examining materials and products in early developmental stage. However, a number of studies pointed out its disadvantage that it overreacts to comedogenic materials. The purpose of this study was to find the most appropriate region for evaluating comedogenicity in human skin.

METHODS

Sixty-six female subjects (age 32.48 ± 10 years; range 20-52 years) with mild to moderate facial acne lesions were included in this study. The whole face, upper chest, and back of volunteers were photographed. Lesion (closed and open comedones) counting, instrumentation of sebum secretion level, and analysis of porphyrin number were performed. The entire study was performed under environmental conditions of specific relative temperature and humidity, controlled and maintained identically for each volunteer.

RESULTS

In case of closed comedone, forehead showed a significant correlation with frontal cheek, lateral cheek, chin, and upper back. Meanwhile, significant correlations were observed between frontal cheek and chin as well as lateral cheek and chest. As for open comedone, forehead showed a significant correlation with chin site. A significant correlation was also observed between front cheek and lateral cheek as well as between upper chest and back. Analyzing the correlation between the occurrence of comedones and sebum in each region, a significant correlation between closed comedone and sebum was observed in frontal and lateral cheek. Analyzing the correlation between the occurrence of comedones and porphyrine in each region, a significant correlation between open comedone and porphyrin was observed in chin.

CONCLUSION

When evaluating the comedogenicity of cosmetics ingredients or products, this study recommends using both of the methods of testing on back and directly testing on face according to the characteristics of the materials. In case of mild potent ingredients or products in particular, verification through usability test that the directly test on face will help securing reliability.

A dermocosmetic containing bakuchiol, Ginkgo biloba extract and mannitol improves the efficacy of adapalene in patients with acne vulgaris: result from a controlled randomized trial

BACKGROUND

Acne vulgaris is an inflammatory disorder of the pilosebaceous unit.

AIM

To confirm that BGM (bakuchiol, Ginkgo biloba extract, and mannitol) complex increases the established clinical efficacy of adapalene 0.1% gel in patients with acne.

METHODS

A clinical trial was conducted in acne patients. A total of 111 subjects received adapalene 0.1% gel and BGM complex or vehicle cream for 2 months. Assessments comprised Investigator Global Assessment (IGA), global efficacy, seborrhea intensity, inflammatory and non-inflammatory lesions, and subject perception, as well as overall safety and local tolerance and quality of life.

RESULTS

At the end of the trial, inflammatory and non-inflammatory lesions, IGA, global efficacy, and seborrhea intensity had significantly improved in both treatment groups. Differences were statistically significant (P<0.05) in favor of BGM complex for inflammatory lesions as well as IGA and seborrhea intensity. Global efficacy assessments and subject perception confirmed the superiority of BGM complex-including treatment over the comparative combination. Quality of life had improved more with the active combination than with the vehicle combination. In the active group, four subjects had to interrupt temporarily BGM complex and 12 adapalene compared to seven subjects interrupting the vehicle and eleven adapalene in the vehicle group. One subject withdrew from the trial due to an allergy to adapalene. The majority of all events were mild.

CONCLUSION

BGM complex improves the treatment outcome of adapalene 0.1% gel in patients with acne vulgaris. Overall, safety and local tolerance of BGM complex were good.

Oxidization of squalene, a human skin lipid: a new and reliable marker of environmental pollution studies

A review of the oxidization of squalene, a specific human compound produced by the sebaceous gland, is proposed. Such chemical transformation induces important consequences at various levels. Squalene by-products, mostly under peroxidized forms, lead to comedogenesis, contribute to the development of inflammatory acne and possibly modify the skin relief (wrinkling). Experimental conditions of oxidation and/or photo-oxidation mechanisms are exposed, suggesting that they could possibly be bio-markers of atmospheric pollution upon skin. Ozone, long UVA rays, cigarette smoke… are shown powerful oxidizing agents of squalene. Some in vitro, ex vivo and in vivo testings are proposed as examples, aiming at studying ingredients or products capable of boosting or counteracting such chemical changes that, globally, bring adverse effects to various cutaneous compartments.

Acne is an inflammatory disease and alterations of sebum composition initiate acne lesions

Hyperseborrhoea has been considered as a major aetiopathogenetic factor of acne. However, changes in sebaceous gland activity not only correlate with seborrhoea but also with alterations in sebum fatty acid composition. Current findings indicate that sebum lipid fractions with proinflammatory properties and inflammatory tissue cascades are associated in the process of the development of acne lesions. The oxidant/antioxidant ratio of the skin surface lipids and alterations of lipid composition are the main players in the induction of acne inflammation. Nutrition may influence the development of seborrhoea, the fractions of sebum lipids and acne. Acne is an inflammatory disease probably triggered, among others, by proinflammatory sebum lipid fractions.

Sebusuppressive efficacy of the antioxidant bis-ethylhexyl hydroxydimethoxy benzylmalonate in the treatment of oily and blemished skin

The aim of this study was to assess the effect of 2% bis-ethylhexyl hydroxydimethoxy benzylmalonate (HDBM; RonaCare(®) AP) as an active ingredient in the treatment of oily and blemished skin. This study was carried out as a randomized, placebo-controlled, single-blind study on 44 test subjects with blemished skin over a period of 8 weeks. Sebum measurement, skin inflammation/redness scoring by an expert, photographic documentation and a self-assessment with regard to skin improvement, and tests of skin compatibility and galenic characteristics were performed. Treatment with 2% HDBM resulted in a significant reduction of sebum excretion and showed efficacy against inflamed/red lesions also shown by photographic documentation. Efficacy and galenic performance of 2% HDBM were judged to be superior to the placebo emulsion. Additionally, 2% HDBM was well tolerated; approximately 80% of the test subjects rated the compatibility as good to very good.

Sebum analysis of individuals with and without acne

A pilot study was conducted to compare lipid components of sebum from unaffected and acne-affected individuals. Nine males, 15-20 years old, with no acne, or with moderate to severe acne, were recruited. Facial images were taken with regular, polarized and fluorescent lights for each subject. Skin surface lipids were analyzed following collection of sebum using sebutapes. As expected, the subjects with acne had more (59%) sebum than the control subjects. Free fatty acids were the only lipid group that was reduced in the sebum of acne subjects. The specific lipid that differed the most between the two groups was squalene, which was upregulated in acne subjects by 2.2-fold on a quantitative basis. Squalene also represented a significantly greater proportion of the total sebaceous lipids in acne patients compared to controls (20% vs. 15%). The increase in the amount of squalene could represent a lipid marker for acne prone skin.

Epidermal surface lipids

A layer of lipids, which are of both sebaceous and keratinocyte origin, covers the surface of the skin. The apparent composition of surface lipids varies depending on the selected method of sampling. Lipids produced by the epidermal cells are an insignificant fraction of the total extractable surface lipid on areas rich in sebaceous glands. Due to the holocrine activity of the sebaceous gland, its product of secretion (sebum) is eventually released to the surface of the skin and coats the fur as well. Lipids of epidermal origin fill the spaces between the cells, like mortar or cement. The sebaceous lipids are primarily non polar lipids as triglycerides, wax esters and squalene, while epidermal lipids are a mixture of ceramides, free fatty acids and cholesterol. The composition of the sebaceous lipids is unique and intriguing and elevated sebum excretion is a major factor involved in the pathophysiology of acne. Recent studies have elucidated the roles that epidermal surface lipids have on normal skin functions and acne.

Sebaceous gland lipids

The principal activity of mature sebaceous glands is producing and secreting sebum, which is a complex mixture of lipids. Sebum composition is different among species and this difference is probably due to the function that sebum has to absolve. In human sebum there are unique lipids, such as squalene and wax esters not found anywhere else in the body nor among the epidermal surface lipids. Moreover, they correspond to major components supplying the skin with protection. However, the ultimate role of human sebum, as well the metabolic pathways regulating its composition and secretion rate, are far from a complete understanding. Increased sebum secretion is considered, among all features, the major one involved in the pathophysiology of acne. Along with increased sebum secretion rate, quali- and quantitative modifications of sebum are likely to occur in this pathology. Understanding the factors and mechanisms that regulate sebum production is needed in order to identify new targets that can be addressed to achieve a selective modulation of lipid biosynthesis as a novel therapeutic strategy to correct lipid disregulations in acne and other disorders of the pilosebaceous unit.

Acne and smoking

BACKGROUND.: Post-adolescent acne is an inflammatory disorder, whose cause is unknown. Contrasting data are available on correlation between acne and smoking habit. OBJECTIVES.: To verify the frequency of clinically non-inflammatory (atypical) post-adolescent acne (APAA) among women, a possible correlation with cigarette smoking, possible differences in sebum composition in a group of female smokers with acne compared to healthy smokers and non-smokers. METHOD AND RESULTS.: 1046 randomly selected women (25-50-years-old) participated at the study. In 60 selected female subjects we analyzed sebum composition for alpha-tocopherol, squalene and squalene monohydroperoxide. We found a high prevalence of APAA among women (74.6%), a strong correlation with smoking habit (p < 0.0001), as well as an increase in the grade of sebum peroxidation (p < 0.05) with a reduction in vitamin E (p = 0.02), in the subjects with acne compared to the controls. CONCLUSIONS.: Clinical evidence and experimental data showed a straight correlation between smoking habit and post-pubertal acne in which the clinically non-inflammatory type-APAA-is the most frequent. In the more severe cases we could consider APAA as a new entity (smoker's acne).

Clinical implications of lipid peroxidation in acne vulgaris: old wine in new bottles

Acne vulgaris is a common dermatological disorder, one that is frequently associated with depression, anxiety and other psychological sequelae. In recent years there has been an increasing focus on the extent to which oxidative stress is involved in the pathophysiology of acne. Emerging studies have shown that patients with acne are under increased cutaneous and systemic oxidative stress. Indeed, there are indications that lipid peroxidation itself is a match that lights an inflammatory cascade in acne. The notion that lipid peroxidation is a 'starter gun' in acne is not a new one; here we review the nearly 50-year-old lipid peroxidation theory and provide a historical perspective to the contemporary investigations and clinical implications.

In addition, we present a novel hypothesis in which lipid peroxidation may be priming an increased susceptibility to co-morbid depression and anxiety in those with acne. The emerging research on the systemic burden of oxidative stress in acne sheds further light on the brain-skin axis. The recent findings also suggest potential avenues of approach for the treatment of acne via specific nutrients, dietary modifications, oral and topical interventions.

[The sebaceous gland]

The development and function of the sebaceous gland in the fetal and neonatal periods appear to be regulated by maternal androgens and by endogenous steroid synthesis, as well as by other morphogens. The most apparent function of the glands is to excrete sebum. A strong increase in sebum excretion occurs a few hours after birth; this peaks during the first week and slowly subsides thereafter. A new rise takes place at about age 9 years with adrenarche and continues up to age 17 years, when the adult level is reached. The sebaceous gland is a target organ but also an important formation site of hormones, and especially of active androgens. Hormonal activity is based on an hormone (ligand)-receptor interaction, whereas sebocytes express a wide spectrum of hormone receptors. Androgens are well known for their effects on sebum excretion, whereas terminal sebocyte differentiation is assisted by peroxisome proliferator-activated receptor ligands. Estrogens, glucocorticoids, and prolactin also influence sebaceous gland function. In addition, stress-sensing cutaneous signals lead to the production and release of corticotrophin-releasing hormone from dermal nerves and sebocytes with subsequent dose-dependent regulation of sebaceous nonpolar lipids. Among other lipid fractions, sebaceous glands have been shown to synthesize considerable amounts of free fatty acids without exogenous influence. Atopic dermatitis, seborrheic dermatitis, psoriasis and acne vulgaris are some of the disease on which pathogenesis and severity sebaceous lipids may or are surely involved.

Surface lipids as multifunctional mediators of skin responses to environmental stimuli

Skin surface lipid (SSL) film is a mixture of sebum and keratinocyte membrane lipids, protecting skin from environment. Its composition is unique for the high percentage of long chain fatty acids, and of the polyterpenoid squalene, absent in other human tissues, and in non-human Primates sebum. Here, the still incomplete body of information on SSL as mediators of external chemical, physical, and microbial signals and stressors is revised, focusing on the central event of the continuous oxidative modification induced by the metabolic activity of residential and pathological microbial flora, natural or iatrogenic UV irradiation, exposure to chemicals and cosmetics. Once alpha-tocopherol and ubiquinol-10 antioxidant defences of SSL are overcome, oxidation of squalene and cholesterol gives rise to reactive by-products penetrating deeper into skin layers, to mediate local defensive inflammatory, photo-protective, immune reactions or, at higher concentrations, inducing local but also systemic immune depression, ultimately implicating skin cancerogenesis. Qualitative modifications of SSL represent a pathogenetic sign of diagnostic value in dermatological disorders involving altered sebum production, like pytiriasis versicolor, acne, atopic or seborrheic dermatitis, as well as photo-aging. Achievements of nutriceutical interventions aimed at restoring normal SSL composition and homeostasis are discussed, as feasible therapeutic goals and major means of photo-protection.

Lipid mediators in acne

Multiple factors are involved in acne pathogenesis, and sebum secretion is one of the main ones. The role sebum plays in acne development has not been completely elucidated yet; however, increasing amounts of data seem to confirm the presence of alterations in sebum from acne patients. Altered ratio between saturated and unsaturated fatty acids has been indicated as an important feature to be considered in addition to the altered amount of specific fatty acids such as linoleic acid. Furthermore, particular attention has been focused on squalene peroxide that seems to be able to induce an inflammatory response beyond cytotoxicity and comedones formation. Moreover, recent data suggest that lipid mediators are able to interfere with sebocytes differentiation and sebogenesis through the activation of pathways related to peroxisome proliferators-activated receptors. Understanding the factors and mechanisms that regulate sebum production is needed in order to identify novel therapeutic strategies for acne treatment.

gamma-Tocotrienol reduces squalene hydroperoxide-induced inflammatory responses in HaCaT keratinocytes

Squalene hydroperoxide (SQ-OOH), the primary peroxidation product of squalene (SQ), accumulates at the surface of sunlight-exposed human skin. There are however only a few studies on the pathogenic actions (i.e., inflammatory stimuli) of SQ-OOH. Here, we evaluated whether SQ-OOH induced inflammatory responses in immortalized human keratinocytes (HaCaT). We found that SQ-OOH caused an increase in the expression of inflammatory genes such as the interleukins as well as cyclooxygenase-2 (COX-2). In concordance with the upregulation of COX-2 mRNA, SQ-OOH enhanced reactive oxygen species generation, nuclear factor kappa B activation, COX-2 protein expression, and prostaglandin E2 production. Therefore, the pro-inflammatory effects of SQ-OOH may be mediated in part via COX-2. On the other hand, gamma-tocotrienol (gamma-T3, an unsaturated form of vitamin E) was found to ameliorate the SQ-OOH actions. These results suggest that SQ-OOH induces inflammatory responses in HaCaT, implying that SQ-OOH plays an important role in inflammatory skin disorders. As a preventive strategy, inflammation could be reduced via the use of gamma-T3.

Squalene as a target molecule in skin hyperpigmentation caused by singlet oxygen

Based on our previous finding (Biochem. Biophys. Res. Commun., 223, 578-582, 1996) of singlet oxygen generation from coproporphyrin excreted on the skin surface from Propionibacterium acnes, we hypothesized that singlet oxygen formed in this way under UV exposure would promote peroxidation of skin surface lipids. We found that squalene was oxidized efficiently by singlet oxygen derived from coproporphyrin under UV exposure, and that the rate constant of squalene peroxidation by singlet oxygen was ten-fold higher than that of other skin surface lipids examined. The reaction was promoted more efficiently by UVA than by UVB. Furthermore, we found that topical application of squalene peroxide induced skin hyperpigmentation through increasing prostaglandin E(2) release from keratinocytes in guinea pigs. These results suggest that squalene peroxide formation by singlet oxygen plays a key role in photo-induced skin damage.

Antioxidant Activities of Essential Oil Mixtures toward Skin Lipid Squalene Oxidized by UV Irradiation

Antioxidant activities of essential oil mixtures--thyme or clove leaf with cinnamon leaf, rose, or parsley seed--toward skin lipid, squalene oxidized by UV irradiation were investigated using the malonaldehyde/gas chromatography assay. At all concentrations (50, 100, or 500 mug/mL) tested, thyme oil mixed with 500 mug/mL clove oil showed over 90% inhibitory effect against malonaldehyde formation. The order of potency of all oils mixed together at 500 mug/mL was thyme/clove leaf (93%) > clove leaf/parsley seed = clove leaf /rose (87%) > thyme/parsley seed (83%) > clove leaf/cinnamon leaf (77%) > thyme/parsley seed (71%) > thyme/cinnamon leaf (7%). In comparison, the inhibitory activities of 500 microg/mL of BHT or alpha-tocopheroltoward malonaldehyde formation were 85% and 76%, respectively. Pro-oxidant effects were observed for some mixtures of thyme with cinnamon leaf or rose oils. The potent antioxidant effects resulting from a mixture of thyme and clove leaf oils may be due to the presence of thymol and eugenol.

Ion-trap tandem mass spectrometric analysis of squalene monohydroperoxide isomers in sunlight-exposed human skin

We previously discovered that squalene monohydroperoxide (SQ-OOH) was produced on human forehead skin and suggested that skin squalene (SQ) may be the principal target lipid for oxidative stress (e.g., sunlight exposure). Because of its six double bonds, SQ peroxidation can yield various positional hydroperoxide isomers. However, the structural characterization of skin SQ-OOH isomers has never been reported. Here, we prepared pure SQ-OOH isomers and developed an analytical method for SQ-OOH isomers using a quadrupole/linear ion-trap mass spectrometer (QTRAP) MS/MS system. Collision-induced dissociation produced specific fragment ions for each SQ-OOH isomer, which permitted discrimination between SQ-OOH isomers by multiple reaction monitoring (MRM). When lipid extract from human forehead skin was subjected to LC-MS/MS with MRM, individual SQ-OOH isomers could be separated and detected with a sensitivity of 0.05 ng/injection. The total concentration of SQ-OOH isomers in forehead skin was approximately 956 microg/g skin lipids, but it increased up to 2,760 microg/g skin lipids after 3 h of sunlight exposure. The LC-MS/MS method was useful for investigating the peroxidation mechanisms of SQ as well as SQ-OOH-mediated skin disorders.

Interplay among oxidants, antioxidants, and cytokines in skin disorders: Present status and future considerations

The pathogenicity of skin disorders involves a complexity of physiological, immunological, environmental, and genetic phenomena. This review focuses on cross-talks between two main agents, the oxidants and cytokines network, which have recently been found to play important roles in the pathophysiology of a large variety of skin disorders, including carcinogenesis, UVB irradiation damages, inflammatory processes, and a series of diseases such as, psoriasis, pyoderma gangrenosum, atopic dermatitis, irritant contact dermatitis, and bacterial skin infections. In particular the review discusses the question how an interplay between oxidants and cytokines might be beneficial in wound-healing and in therapeutic strategies in clinical settings. These involve topical applications and oral administration of antioxidant and inflammatory-cytokines-neutralizing antibodies. Monitoring cytokine expression in skin disorders (inflammatory versus anti-inflammatory, or Th1 versus Th2 types of cytokines) will definitely help to evaluate the severity of injury, its type, and its role in therapy. Furthermore, it is expected that future studies should explore the possible roles of the synergistic interactions between antioxidants and cytokines and their impact on the Th1/Th2 cytokine networks balances.

[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.

Peroxidated Squalene Induces the Production of Inflammatory Mediators in HaCaT Keratinocytes: A Possible Role in Acne Vulgaris

In order to investigate whether products derived from the oxidation of sebum can be responsible for the induction of inflammatory processes, HaCaT keratinocytes were treated with peroxidated squalene. NF-kappaB activation, secretion, and expression of IL-6, as well as peroxisome proliferator-activated receptor alpha (PPARalpha) mRNA and protein levels, were measured at the end of the treatment and after 24 and 48 hours of recovery. Squalene peroxidation products were administered in amounts able to elicit significant hyperproliferation and to induce lipoxygenase (LOX) activity. The results showed an early activation of NF-kappaB followed by an increase in PPARalpha mRNA and protein levels. Moreover, squalene peroxides induced an initial upregulation of IL-6 production and secretion that was counteracted by PPARalpha activation, as suggested by the subsequent decrease of NF-kappaB nuclear translocation and IL-6 levels. Inflammatory processes play an important role in the development of acne vulgaris. In combination with our own previous findings, which indicated an association between LOX stimulation and increased percentage of proinflammatory lipids in acne as well as a correlation between increased cytokine levels in the infundibulum, pilosebaceous duct hyperkeratinization, and augmented sebogenesis, the present data further support the involvement of lipid peroxides, in particular squalene peroxides, in establishing an inflammatory process in acne.

Oral supplementation with all-Rac- and RRR-alpha-tocopherol increases vitamin E levels in human sebum after a latency period of 14-21 days

In human skin, highest alpha-tocopherol levels are found in facial sebum. We hypothesized that the bioavailability of vitamin E in human skin is, at least in part, dependent on sebaceous gland secretion. To test this, 24 volunteers were subjected to a randomized daily supplementation with either 400 mg RRR-alpha-tocopheryl acetate (RRR-alpha-toc) or 400 mg all-rac-alpha-tocopheryl acetate (all-rac-alpha-toc) for 14 days. Fasting blood samples, facial sebum samples, and lower-arm skin-surface lipids (SSL) were taken at time-points between 0-21 days. Samples were analyzed by HPLC for alpha-tocopherol and squalene concentrations. Increased serum alpha-tocopherol levels were detectable as early as 12 h after supplementation of RRR-alpha-toc or all-rac-alpha-toc and peaked on day 7. No significant changes were observed in lower-arm SSL. Remarkably, while unchanged until day 14, alpha-tocopherol sebum levels were increased on day 21 in both the RRR-alpha-toc and the all-rac-alpha-toc group by 87% and 92%, respectively. With respect to dietary supplementation of vitamin E and its bioavailability in human skin, these results suggest that (1) sebaceous gland secretion is a relevant delivery mechanism; (2) the bioavailabilities of RRR-alpha-toc and the all-rac-alpha-toc are similar; and (3) significant accumulation requires a daily supplementation period of at least 2-3 weeks.

Vitamin E Delivery to Human Skin by a Rinse-Off Product: Penetration of α-Tocopherol versus Wash-Out Effects of Skin Surface Lipids

alpha-Tocopherol, the major biologically active form of vitamin E, represents a frequently added lipophilic compound of skin care products. Despite its emerging use in rinse-off formulations, little is known on its efficacy with respect to its deposition or its antioxidant potential in human skin. The objective of this study was to investigate whether the single use of an alpha-tocopherol-enriched rinse-off product provides effective deposition of alpha-tocopherol on human stratum corneum. To test this, forearm skin of 13 volunteers was washed either with an alpha-tocopherol-enriched rinse-off product (test product, TP) or with an alpha-tocopherol-free vehicle control (control product, CP) (contralateral arm) using a standardized wash protocol. Thereafter, skin surface lipids were extracted with pure ethanol after the wash procedure as well as after 24 h. Additionally, one group of volunteers was subjected to irradiation of their forearms with low-dose UVA (8 J/cm(2)) prior to lipid extraction. Skin lipid extracts were analyzed by high performance liquid chromatography using electrochemical detection for vitamin E and UV detection for squalene (SQ) and squalene monohydroperoxide. The results of this in vivo study demonstrated that (1) while CP treatment lowers, TP treatment strongly increases alpha-tocopherol levels of skin barrier lipids; (2) increased vitamin E deposition levels were maintained for a period of at least 24 h, and (3) TP treatment significantly inhibited photooxidation of SQ. In conclusion, the use of alpha-tocopherol-enriched rinse-off products may help to maintain the integrity of the skin barrier by providing protection against photooxidative stress at the level of skin surface lipids.

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.

Changes in the levels of glutathione after cellular and cutaneous damage induced by squalene monohydroperoxide

Squalene monohydroperoxide (Sq-OOH), the initial product of ultraviolet-peroxidated squalene, was used to investigate the effect of peroxidative challenge upon the glutathione contents in rabbit ear skin and primary-cultured fibroblasts derived from rabbit ear skin. The cellular reduced glutathione (GSH) contents decreased during 30-minute incubations in vitro with Sq-OOH, and oxidized glutathione (GSSG) was formed concomitantly, indicating that Sq-OOH had a potential for GSH-depleting activity in vitro. When Sq-OOH was applied topically to the skin in vivo, only GSSG contents increased significantly within 30 minutes. Moreover, pretreatment with the GSH depletors, DL-buthionine sulfoximine (BSO) and diethyl maleate (DEM), could potentiate the cytotoxicity and comedogenicity induced by Sq-OOH. These findings suggest that the endogenous antioxidant, glutathione, is quite sensitive to Sq-OOH and may be an important material for protecting cells and/or tissues against the oxidative stress induced by Sq-OOH treatment.