Broad-spectrum sunscreens offer protection against urocanic acid photoisomerization by artificial ultraviolet radiation in human skin

Skin Barrier- and Immune Response-Related Biomarkers of Solar UVR Exposure Comparing Indoor and Outdoor Workers

Outdoor workers have increased risk of developing keratinocyte cancer due to accumulated skin damage resulting from chronic and excessive exposure to UVR. This study aims to identify potential noninvasive biomarkers to assess chronic UVR exposure. We analyzed stratum corneum biomarkers collected from 2 skin locations and 2 occupational groups with contrasting solar UVR exposure: the forehead and retroauricular skin among outdoor workers and indoor workers. Using a linear mixed model adjusting for age and skin phototype, we compared biomarkers between both skin sites in indoor and outdoor workers. We measured markers of the immune response and skin barrier, including cytokines, GFs, 15-hydroxyeicosatetraenoic acid, cis- and trans-urocanic acid, and corneocyte topography, indicated by circular nano objects. Differences between the 2 skin sites were found for cis-urocanic acid, total urocanic acid, IL-1α, IL-1RA, IL-1RA/IL-1α, IL-18, 15-hydroxyeicosatetraenoic acid, CCL4, and circular nano objects. The levels of cis-urocanic acid and CCL4 also differed between indoor and outdoor workers. These findings underscore changes in both immune response and skin barrier induced by UVR. They indicate the potential utility of stratum corneum biomarkers in detecting both chronic UVR exposure in occupational setting and aiding in the development of preventive measures.

Redefine photoprotection: Sun protection beyond sunburn

Excessive exposure to ultraviolet (UV) light leads to acute and chronic UV damage and is the main risk factor for the development of skin cancer. In most countries with western lifestyle, the topical application of sunscreens on UV-exposed skin areas is by far the most frequently used preventive measure against sunburn. Further than preventing sunburns, increasing numbers of consumers are appreciating sunscreens with a medium- to high-level sun protective factor (SPF) as basis for sustainable-skin ageing or skin cancer prevention programs. However, recent investigations indicate that clinically significant DNA damages as well as a lasting impairment of cutaneous immunosurveillance already occur far below the standard of one minimal erythema dose (MED) sunburn level, which contributes to the current discussion of the clinical value of high-protective SPF values. Ex vivo investigations on human skin showed that the application of SPF30 reduces DNA damage for a day long sun exposure (24 MED) drastically by about 53% but is significantly surpassed by SPF100 reducing DNA damage by approx. 73%. Further analysis on different SPF protection levels in UV-exposed cell culture assays focusing on IL-18, cell vitality and cis/trans-urocanic acid support these findings. Whereas SPF30 and SPF50+ sunscreens already offer a solid UVB cover for most indications, our results indicate that SPF100 provides significant additional protection against mutagenic (non-apoptotic-) DNA damage and functional impairment of the cutaneous immunosurveillance and therefore qualifies as an optimized sunscreen for specifically vulnerable patient groups such as immunosuppressed patients, or skin cancer patients.

Stratum corneum biomarkers after in vivo repeated exposure to sub-erythemal dosages of ultraviolet radiation in unprotected and sunscreen (SPF 50+) protected skin

BACKGROUND

There is a need for non-invasive biomarkers to assess in vivo efficacy of protective measures aiming at reducing ultraviolet radiation (UVR) exposure. Stratum corneum (SC) biomarkers showed to be promising markers for internal UVR dose and immune response.

PURPOSE

To establish a dose-response relationship for SC biomarkers and explore their suitability for in vivo assessment of the blocking effect of two sunscreens with a high sun protection factor (SPF) (50+).

METHODS

Twelve volunteers were exposed to a broad-spectrum UVB (280-320 nm), five times a week, during one week. Unprotected back skin was irradiated with 0.24, 0.48, 0.72 and 1.44 standard erythema dose (SED) and sunscreen-protected skin with 3.6 SED. SC samples for determination of the relative amount of cis-urocanic acid (cUCA) and thirteen immunological makers including cytokines and matrix metalloproteinases (MMP) were collected after each irradiation.

RESULTS

cUCA sharply increased after the first irradiation in a dose-dependent fashion. However, it levelled-off after subsequent exposures and reached a plateau for the highest UV-dose after the third irradiation. None of the immunological markers showed dose-dependency. However, MMP-9, IL-1β and CCL27 increased gradually from baseline during repetitive exposures to the highest UV-dose. Assessed from cUCA, both sunscreens blocked >98% of the applied UV-dose.

CONCLUSIONS

cUCA is a sensitive, non-invasive marker of the internal UVR dose enabling in vivo assessment of the blocking effect of high SPF sunscreens in the UVB-region. Immunological SC markers show low sensitivity in detecting immune response at sub-erythemal UVR dosages, suggesting they might be suitable only at higher and/or repetitive UVR exposure.

Protection Against Solar Ultraviolet Radiation in Outdoor Construction Workers: Study Protocol for a Non-randomized Controlled Intervention Study

Introduction: Non-melanoma skin cancer (NMSC) incidence is increasing, and occupational solar exposure contributes greatly to the overall lifetime ultraviolet radiation (UVR) dose. This is reflected in an excess risk of NMSC showing up to three-fold increase in outdoor workers. Risk of NMSC can be reduced if appropriate measures to reduce UVR-exposure are taken. Regular use of sunscreens showed reduced risk of NMSC. However, sun-safety behavior in outdoor workers is poor. The objective of this study is to investigate the effectiveness of an intervention aiming at increasing sunscreen use by construction workers.

Methods: This non-randomized controlled intervention study is comprised of two intervention and two control groups recruited at four different construction sites in the Netherlands. The study population comprises ~200 construction workers, aged 18 years or older, followed during 12 weeks. The intervention consists of providing dispensers with sunscreens (SPF 50+) at construction sites and regular feedback on the application achieved by continuous electronic monitoring. All groups will receive basic information on UV-exposure and skin protection. Stratum corneum (SC) samples will be collected for measurement of biomarkers to assess internal UV-dose. External UV-dose will be assessed by personal UV-sensors worn by the workers during work-shifts in both groups. To detect presence of actinic keratosis (AK) or NMSC, a skin check of body parts exposed to the sun will be performed at the end of the study. The effect of the intervention will be assessed from data on self-reported sunscreen use by means of questionnaires collected on baseline and after 12 weeks of intervention (primary outcome). Levels of SC biomarkers of internal UV-dose, external UV-dose, number of sunburn episodes, and prevalence of NMSC including AK will be assessed as secondary outcomes. The electronically monitored sunscreen consumption will be assessed as process outcome.

Discussion: This study is intended to provide evidence of the effectiveness of a technology-driven intervention to increase sunscreen use in outdoor construction workers. Furthermore, it will increase insight in the UV-protective behavior, external and internal UV-exposure, and the prevalence of NMSC, including AK, in construction workers.

Trial Registration: The Netherlands Trial Register (NTR): NL8462 Registered on March 19, 2020.

Analytical tools for urocanic acid determination in human samples: A review

Urocanic acid is a chromophore found in the skin that has been identified as an important immunosuppressant and carcinogenesis mediator through its photoisomerization from trans to cis form induced by ultraviolet radiation. Research on analytical methods that explore urocanic acid isomerization is indispensable to fully understand the deleterious effects mediated by this biomarker. In this context, the current relevant analytical methods for determination of these isomers in human samples are summarized in this review. The methods presented here are applicable to human samples collected by noninvasive methods (or minimally invasive), encompassing an array of analytical techniques, including high-performance capillary electrophoresis, confocal Raman spectroscopy, gas chromatography, high-performance liquid chromatography, and mass spectrometry, among others. Developed high-performance liquid chromatography methods have proven to be advantageous, allowing noninvasive collections for in vivo analysis and the confocal Raman, specially, for real-time analysis. Among all these methods, high-performance liquid chromatography is the most investigated one with mass spectrometry or ultraviolet detector, and the mass spectrometry detector being the most studied in the last years, demonstrating high sensitivity, very low detection limits, and accurate identification, especially for clinical investigations.

Tape stripping the stratum corneum for biomarkers of ultraviolet radiation exposure at sub-erythemal dosages: a study in human volunteers

PURPOSE

Prevalence of skin cancer is rapidly increasing. There is a need for non-invasive biomarkers to assess efficacy of prevention strategies aiming at reduction of exposure to ultraviolet radiation (UVR). Recently, stratum corneum (SC) biomarkers were applied in various inflammatory skin diseases. Here, we explore their suitability as candidate biomarkers for UVR.

MATERIAL AND METHODS

Twelve volunteers were exposed to a UVB-dose of 0.72 SED, three times a week, during three weeks. As candidate biomarkers, cis-isomers of urocanic acid (cUCA) and 25 immunological mediators were measured in the SC.

RESULTS

Eight immunological markers significantly changed from baseline. Of them, IL-1RA/IL-1α and a placental growth factor (PIGF) showed gradual changes during UVR-exposure (p < 0.01 for linear trend). cUCA increased sharply already after the first exposure, however, reached a plateau in the second week.

CONCLUSIONS

SC represents a promising, non-invasive alternative to skin biopsy in detecting UVR-induced changes. cUCA is the marker of choice for assessment of single UVR-exposure; however, it is less suitable for cumulative UVR-dose. Immunological markers including IL-1RA/IL-1α and PIGF showed gradual changes, and therefore are convenient for monitoring chronic UVR-exposure. These candidate biomarkers might facilitate assessment of the efficacy of preventive measures in the workplace and general population.

Filaggrin - revisited

Profilaggrin (proFLG) and its processing products are critical to the health and appearance of skin. The recent identification of loss-of-function filaggrin (FLG) mutations as a predisposing factor in ichthyosis vulgaris and atopic dermatitis has lead to a resurgent interest in this enigmatic protein. Here, we review the literature on the structure and many functions of proFLG, from its role as a filament-aggregating protein and a source of natural moisturizing factor (NMF), to the more recent discoveries of its role in epidermal barrier formation and its more speculative functions as an antimicrobial and sunscreen. Finally, we discuss the relationship of proFLG with dry skin, the influence of moisturizers on NMF generation and speculate on next generation of FLG research.

Photoprotection against the UVB-induced oxidative stress and epidermal damage in mice using leaves of three different varieties of Lepidium meyenii (maca)

BACKGROUND

Skin exposure to ultraviolet (UV) B radiation leads to epidermal damage and generation of reactive oxygen species. The photoprotective effect of extracts of three varieties of leaves (red, yellow, and black) from maca (Lepidium meyenii), a plant from the Peruvian highlands, was assessed in mouse skin exposed to UVB radiation.

MATERIALS AND METHODS

The hydroalcoholic extracts of three varieties of maca leaves were applied topically to the dorsal skin of young-adult male mice prior to exposition to UVB radiation.

RESULTS

The three varieties had UVA/UVB absorptive properties and presented antioxidant activity, being highest with red maca, followed by black and yellow maca. The three varieties of maca leaves prevented the development of sunburn cells, epidermal hyperplasia, leukocytic infiltration, and other alterations produced by UVB radiation. Mice treated with black maca showed the highest superoxide dismutase levels, and mice treated with black and yellow maca showed higher catalase levels in skin, whereas red maca protected the skin and liver against significant increases in the lipid peroxidation activity observed in the unprotected animals.

CONCLUSION

The presence of significant antioxidant activity and the inhibition of lipid peroxidation suggest that the observed protection could be partly attributable to this mechanism.

Recent advances in urocanic acid photochemistry, photobiology and photoimmunology

Urocanic acid (UCA), produced in the upper layers of mammalian skin, is a major absorber of ultraviolet radiation (UVR). Originally thought to be a 'natural sunscreen', studies conducted a quarter of a century ago proposed that UCA may be a chromophore for the immunosuppression that follows exposure to UVR. With its intriguing photochemistry, its role in immunosuppression and skin cancer development, and skin barrier function, UCA continues to be the subject of intense research effort. This review summarises the photochemical, photobiological and photoimmunological findings regarding UCA, published since 1998.

An Action Spectrum for the Production of cis-Urocanic Acid in Human Skin In Vivo

Urocanic acid (UCA) is present at millimolar concentrations in mammalian epidermis and undergoes photoisomerization from the naturally occurring trans-isomer to the cis-isomer on exposure to ultraviolet radiation (UVR). Cis-UCA causes downregulation of various immune responses in mouse and human experimental models and has been proposed as both a chromophore and a mediator of UV-induced immune suppression. In this study, the wavelength dependence from 260-340 nm for trans to cis-UCA photoisomerization in human skin was analyzed in five healthy volunteers. The resulting action spectrum demonstrated maximal cis-UCA production in the UVB spectral region of 280-310 nm. This spectral peak is red-shifted to longer wavelengths compared with the erythemal action spectrum. The cis-UCA action spectrum can be used to predict the ability of sunscreens to protect against UVR-induced cis-UCA formation and may assist in explaining discrepancies between sunscreens' abilities to protect against erythema and photoimmunosuppression.

Sunburn Cell Formation, Dendritic Cell Migration, and Immunomodulatory Factor Production After Solar-Simulated Irradiation of Sunscreen-Treated Human Skin Explants In Vitro

Using human skin explants, we investigated the effects of two different sunscreen preparations containing a chemical UVB filter alone [sun protection factor (SPF) 5.2] or UVA+UVB filter [SPF 6.2] on sunburn cell formation, dendritic cell (DC) migration, CD86- and CD1a-positive cell number, and tumor necrosis factor alpha (TNFalpha) and interleukin (IL)-1, IL-10, and IL-12 production in the skin after irradiation with different doses of solar-simulated UV radiation. Sunscreen- or placebo-treated skin explants were irradiated with solar-simulated UV radiation at 0.5, 1, and 2 minimal erythematous dose equivalents (MEDE) (as determined in an in vivo human study) multiplied by the SPF of the placebo or sunscreens. After irradiation, skin explants were floated on RMPI medium for 48 h. Cells that had emigrated and the skin explants were histologically analyzed, and the soluble mediators were measured in the supernatants by ELISA. Exposure to UV radiation led to concentration-dependent increases in sunburn cell formation and TNFalpha production but a concentration-dependent decrease in DC migration and CD86- and CD1a-positive cell number in the epidermis. Both chemical sunscreens protected against those alterations. The immunoprotective capacity of the sunscreens correlated with their SPF but was independent of the sunscreens' UVA protection capacity, suggesting that UVA is not a major factor for immunosuppression under the conditions used in the model. UV irradiation did not significantly affect the vitality of emigrated DC; the expression of HLA, CD80, and lag on emigrated cells; the number of CD1a-positive cells in the dermis; or the production of IL-1, IL-10, and IL-12. We conclude that our model may be useful in determining the immunoprotective capacity of sunscreens.

High Ultraviolet A Protection Affords Greater Immune Protection Confirming that Ultraviolet A Contributes to Photoimmunosuppression in Humans

Solar radiation causes immunosuppression that contributes to skin cancer growth. Photoprotective strategies initially focused on the more erythemogenic ultraviolet B. More recently, the relationship of ultraviolet A and skin cancer has received increased attention. We hypothesized that if ultraviolet A contributes significantly to human ultraviolet-induced immune suppression, then increased ultraviolet A filtration by a sunscreen would better protect the immune system during ultraviolet exposure. Two hundred and eleven volunteers were randomized into study groups and received solar-simulated radiation, ranging from 0 to 2 minimum erythema dose, on gluteal skin, with or without sunscreen, 48 h prior to sensitization with dinitrochlorobenzene. Contact hypersensitivity response was evaluated by measuring the increase in skin fold thickness of five graded dinitrochlorobenzene challenge sites on the arm, 2 wk after sensitization. Clinical scoring using the North American Contact Dermatitis Group method was also performed. Solar-simulated radiation dose-response curves were generated and immune protection factor was calculated using a nonlinear regression model. Significance of immune protection between study groups was determined with the Mann-Whitney-Wilcoxon exact test. The sunscreen with high ultraviolet A absorption (ultraviolet A protection factor of 10, based on the in vivo persistent pigment darkening method) and a labeled sun protection factor of 15 demonstrated better immune protection than the product that had a low ultraviolet A absorption (ultraviolet A protection factor of 2) and a labeled sun protection factor of 15. Nonlinear regression analysis based on skin fold thickness increase revealed that the high ultraviolet A protection factor sunscreen had an immune protection factor of 50, more than three times its sun protection factor, whereas the low ultraviolet A protection factor sunscreen had an immune protection factor of 15, which was equal to its labeled sun protection factor. This study demonstrates that ultraviolet A contributes greatly to human immune suppression and that a broad-spectrum sunscreen with high ultraviolet A filtering capacity results in immune protection that exceeds erythema protection. These results show that high ultraviolet A protection is required to protect against ultraviolet-induced damage to cutaneous immunity.

Protection against UV-induced suppression of contact hypersensitivity responses by sunscreens in humans

Both in vivo skin immune responses and the skin's reaction to sun exposure integrate a complex interplay of biologic responses. The complexity and multiplicity of events that occur in the skin during an immune response make it a sensitive indication of both UVB and UVA-induced changes in the skin by sun damage, as well as those changes that are prevented by various sunscreens. Sunscreens are the most effective and widely available intervention for sun damage, other than sun avoidance or clothing. However, sunscreens vary widely in their relative ability to screen various UV waveband components, and their testing has been variably applied to outcomes other than for erythema to determine the sunburn protection factor (SPF), a measure primarily of UVB filtration only. Determination of an immune protection factor (IPF) has been proposed as an alternative or adjunctive measure to SPF, and recent studies show IPF can indeed detect added in vivo functionality of sunscreens, such as high levels of UVA protection, that SPF cannot. Clarification of the definition of IPF, however, is required. Excellent data are available on quantification of the IPF for restoring the afferent or induction arm of contact sensitivity, but other immune parameters have also been measured. Proposed here is nomenclature for whether the IPF is measured using contact sensitivity induction (IPF-CS-I), contact sensitivity elicitation (IPF-CS-E), delayed-type hypersensitivity elicitation (IPF-DTH-E), antigen-presenting cell function (IPF-APC-FXN) or numbers (IPF-APC-#), and cytokine modification such as IL-10 (i.e. IPF-cyto-IL-10). Similar nomenclatures could be used for other measures of skin function protection (i.e. DNA damage, p53 induction, oxidation products, etc.). A review of in vivo human studies, in which sunscreens are used to intervene in a UV-induced modulation of immune response, cells or cytokines, highlights the technical variables and statistical approaches which must also be standardized in the context of an IPF for regulatory or product claim purposes. Development of such IPF standards would allow the integration of both UVB and nonUVB (UVA, blue and possible IR) solar waveband effect-reversals, could be applied to integrate effects of other ingredients with protective function (i.e. antioxidants, retinoids, or other novel products), and would spur development of more advanced and complete protection products.

Photoprotection by sunscreens

Ultraviolet (UV) radiation is an etiologic factor for the development of nonmelanoma skin cancers and also possibly melanoma. The American Academy of Dermatology currently recommends the daily use of sunscreens with a sun protection factor (SPF) of 15 or above. The SPF numerical rating is an in vivo, standardized measure of sunscreen efficacy. SPF assessment predominantly reflects the ability of a product to screen UVB radiation. The physical and biologic properties of UVA radiation are relevant as UVA contributes to photoimmunosuppression and photocarcinogenesis. An ideal sunscreen assessment system would address both UVB and UVA protection. However, the SPF rating should continue to serve as the major determinant of sunscreen efficacy.