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Irradiation of Skin with Visible Light Induces Reactive Oxygen Species and Matrix-Degrading Enzymes

  • Frank Liebel
    Affiliations
    Preclinical Pharmacology, Johnson & Johnson Skin Research Center, CPPW, a Unit of Johnson & Johnson Consumer Companies, Skillman, New Jersey, USA
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  • Simarna Kaur
    Affiliations
    Preclinical Pharmacology, Johnson & Johnson Skin Research Center, CPPW, a Unit of Johnson & Johnson Consumer Companies, Skillman, New Jersey, USA
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  • Eduardo Ruvolo
    Affiliations
    Measurement Sciences, Johnson & Johnson Skin Research Center, CPPW, a Unit of Johnson & Johnson Consumer Companies, Skillman, New Jersey, USA
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  • Nikiforos Kollias
    Affiliations
    Measurement Sciences, Johnson & Johnson Skin Research Center, CPPW, a Unit of Johnson & Johnson Consumer Companies, Skillman, New Jersey, USA
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  • Michael D. Southall
    Correspondence
    Preclinical Pharmacology, Johnson & Johnson Skin Research Center, CPPW, a Unit of Johnson & Johnson Consumer Companies, 199 Grandview Road, Skillman, New Jersey 08558, USA.
    Affiliations
    Preclinical Pharmacology, Johnson & Johnson Skin Research Center, CPPW, a Unit of Johnson & Johnson Consumer Companies, Skillman, New Jersey, USA
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      Daily skin exposure to solar radiation causes cells to produce reactive oxygen species (ROS), which are a primary factor in skin damage. Although the contribution of the UV component to skin damage has been established, few studies have examined the effects of non-UV solar radiation on skin physiology. Solar radiation comprises <10% of UV, and thus the purpose of this study was to examine the physiological response of skin to visible light (400–700nm). Irradiation of human skin equivalents with visible light induced production of ROS, proinflammatory cytokines, and matrix metalloproteinase (MMP)-1 expression. Commercially available sunscreens were found to have minimal effects on reducing visible light–induced ROS, suggesting that UVA/UVB sunscreens do not protect the skin from visible light–induced responses. Using clinical models to assess the generation of free radicals from oxidative stress, higher levels of free radical activity were found after visible light exposure. Pretreatment with a photostable UVA/UVB sunscreen containing an antioxidant combination significantly reduced the production of ROS, cytokines, and MMP expression in vitro, and decreased oxidative stress in human subjects after visible light irradiation. Taken together, these findings suggest that other portions of the solar spectrum aside from UV, particularly visible light, may also contribute to signs of premature photoaging in skin.

      Abbreviations

      IR
      infrared
      MMP
      matrix metalloproteinase
      ROS
      reactive oxygen species

      Introduction

      The contribution of the UV (290–400nm) component of solar irradiation on the skin has been well studied; more recently, studies have begun to explore the effects of non-UVR on skin physiology. The spectral distribution of the solar energy at the sea level comprises roughly 3–7% of UVR (290–400nm), 44% of visible light (400–700nm), and 53% of infrared (IR) radiation (700–1440nm;
      • Frederick J.E.
      • Snell H.E.
      • Haywood E.K.
      Solar ultraviolet radiation at the earth's surface.
      ). The effects of UVR are well documented. UVR from sunlight normally consists of three wavelength regions: UVC (which is absorbed by ozone in the atmosphere), UVB, and UVA. Energy from the shorter-wavelength UVB is absorbed in greater amounts by the epidermis and by keratinocyte DNA, compared with the energy from UVA, which penetrates more deeply into the dermal layers of the skin. Visible and IR light wavelengths penetrate deep into the dermis and have been thought to, following absorption, only produce heat. In contrast to the extensive research on the damaging effect of UV, few studies have looked at the effects of visible light on skin.
      It has been known for decades that reactive oxygen species (ROS) produced in the skin following UV irradiation are key mediators of oxidative damage to the skin. Cell damage from UV also occurs through peroxidation of membrane lipids via generation of lipid peroxides. UV irradiation results in the rapid depletion of several endogenous skin enzymes and antioxidants such as glutathione reductase and catalase, as well as glutathione, tocopherol, and ubiquinone. Exposure to UV has also been shown to induce proinflammatory cytokines, such as IL-1α, and matrix metalloproteinases (MMPs) in skin cells such as keratinocytes and fibroblasts (
      • Wlaschek M.
      • Heinen G.
      • Poswig A.
      • et al.
      UVA-induced autocrine stimulation of fibroblast-derived collagenase/MMP-1 by interrelated loops of interleukin-1 and interleukin-6.
      ;
      • Wan Y.
      • Belt A.
      • Wang Z.
      • et al.
      Transmodulation of epidermal growth factor receptor mediates IL-1 beta-induced MMP-1 expression in cultured human keratinocytes.
      ). Activation of MMPs leads to the breakdown of collagen, the major structural component of skin, and also inhibits new collagen synthesis. Induction of inflammatory cytokines by UV, leading to overall skin inflammation, is another significant contributor to the photoaging process in skin. In addition to UV, IR has also been shown to cause oxidative stress to skin; exposing human fibroblasts to IR led to increased ROS formation, and enhanced expression of MMPs (
      • Schroeder P.
      • Lademann J.
      • Darvin M.E.
      • et al.
      Infrared radiation-induced matrix metalloproteinase in human skin: implications for protection.
      ). Additional studies exposed photoprotected sites of healthy human skin to solar relevant doses of IR radiation, and observed that the MMP-1 expression in the dermis, but not the epidermis, was upregulated in 80% of the tested individuals (
      • Cho S.
      • Lee M.J.
      • Kim M.S.
      • et al.
      Infrared plus visible light and heat from natural sunlight participate in the expression of MMPs and type I procollagen as well as infiltration of inflammatory cell in human skin in vivo.
      ).
      Although sunlight comprises up to 44% of visible light, few studies have sought to determine the effects of visible light on skin. Commercial sunscreens are designed to only block wavelengths up to 380nm, and thus skin with topical sunscreens is not protected from the effects of visible light. In the current study, we examined the effect of visible light on the ROS, and MMP responses in skin in vitro. We report that visible light can induce significant ROS production, and this ROS mediates the release of proinflammatory cytokines and MMP expression. Skin is exposed to visible light for substantial durations of the day, and as skin contains several chromophores for visible light the cumulative effects of visible light could result in skin damage, which may contribute to premature skin aging.

      Results

      Exposure to visible light induces increases in epidermal ROS, cytokine, and MMP production

      To assess the role of visible light on skin, human epidermal equivalents were exposed to a dose–response of visible light, and the production of ROS, inflammatory cytokines, and MMPs were determined. Visible light induced a dose-dependent increase in intracellular hydrogen peroxide formation, with visible light doses of 65, 130, and 180Jcm-2 increasing ROS by 5-, 9-, and 18-fold, respectively (Figure 1a). In comparison, a dose of 6.0Jcm-2 from a solar simulator increased ROS by 19-fold. Visible light was also found to increase the release of proinflammatory cytokines from epidermal equivalents. IL-1α release was increased 1.1-, 1.5-, and 2.5-fold after exposure to visible light doses of 65, 130, and 180Jcm-2, respectively, with 6.0Jcm-2 from a solar simulator increasing IL-1α release by 2.8-fold (Figure 1b). A similar effect was seen with release of IL-1 receptor antagonist, IL-6, GM-CSF, and IL-8 (data not shown). In contrast, UV was found to increase the release of tumor necrosis factor-α (TNFα), whereas visible light, even at doses that induced other proinflammatory mediators, did not increase TNFα release. MMP release was also increased after exposure to visible light. MMP-1 release was increased by 2-fold from visible light doses, compared with 2.8-fold by solar simulator (Figure 1c), and MMP-9 release was similarly increased by approximately 2-fold from visible light doses.
      Figure thumbnail gr1
      Figure 1Human epidermal skin equivalents were exposed to the indicated dose of visible light or UVA/UVB, and reactive oxygen species, proinflammatory cytokine, or matrix metalloprotease (MMP) production was measured. Exposure to visible light significantly increased (a) reactive oxygen species, (b) IL-1α, and (c) MMP-1 production in a dose-dependent manner. An asterisk indicates a significant difference (P<0.05) compared with unexposed skin using analysis of variance with Newman–Keuls post hoc test.

      Exposure to visible light induces the EGFR–p42/44 MAPK pathway

      The EGFR/extracellular signal–regulated kinase pathway has been shown to be activated by UV irradiation in keratinocytes and other cell types.2 To examine the effects of visible light on this pathway, primary human keratinocytes were exposed to UV solar light or dose–response of visible light for specific periods of time. Treatment with the positive control (TNFα) and exposure to UV light or visible light (65–180Jcm-2) resulted in increased phosphorylation of the EGFR detected as phospho-tyrosine and the downstream marker of proliferation p42/44 MAPK, suggesting activation of the EGFR–ERK pathway (Figure 2a and b). Total EGFR and ERK were also measured to show uniform protein loading. Pretreatment of keratinocytes with 100nM of the specific EGFR inhibitor, Tyrphostin (AG 1478), for 20minutes before irradiation blocked the downstream effects of visible light on the induction of ERK (Figure 2c), suggesting that the increased phosphorylation of ERK resulted from the intermediate activation of EGFR. These results demonstrate that visible light can induce activation of the EGFR pathway in keratinocytes in a manner similar to UV.
      Figure thumbnail gr2
      Figure 2Primary human keratinocytes exposed to visible light were evaluated for the levels of EGFR and ERK activation. (a, b) Primary human keratinocytes were either left untreated, treated with the positive control tumor necrosis factor-α (TNFα; 100ngml-1), or exposed to UV solar light or visible light for calculated periods of time. The sham group was incubated at room temperature for an equivalent amount of time. Post exposure, the cells were incubated at 37°C for 20minutes, followed by cell lysis. Western blotting with phospho-tyrosine (molecular weight corresponding to EGFR) and phospho-extracellular signal–regulated kinase (ERK) antibodies demonstrated activation of the EGFR–ERK pathway by multiple doses of visible light. (c) Pretreatment with the selective EGFR inhibitor, Tyrphostin (100nM), for 20minutes before irradiation blocked the downstream effects of visible light on the induction of ERK.

      Visible light does not induce thymine dimer formation

      UV exposure is widely known to cause DNA damage in skin, including the formation of thymine (T–T) dimers (
      • Sinha R.P.
      • Hader D.P.
      UV-induced DNA damage and repair: a review.
      ;
      • Marrot L.
      • Meunier J.R.
      Skin DNA photodamage and its biological consequences.
      ). To investigate whether visible light also leads to thymine dimer formation, skin equivalents were treated with visible light or the positive control (UV) and stained for T–T dimer formation. Whereas UV led to strong induction of T–T dimers, visible light did not result in T–T dimer formation even at higher doses of visible light (Figure 3).
      Figure thumbnail gr3
      Figure 3Human epidermal skin equivalents were either left untreated or exposed to UV or visible light for calculated periods of time and stained for T–T dimer formation. Exposure to UV led to induction of T–T dimers, whereas visible light did not have the same effect. Bar=50μm.

      Antioxidants reduce the ROS, cytokine, and MMP production induced by visible light

      To determine whether antioxidants could reduce the damage caused by visible light, an antioxidant combination of Feverfew (Tanacetum parthenium) extract, Soy (Glycine soja) extract, and Gamma Tocopherol were combined into a UVA/UVB Sunscreen and the effect of visible light on ROS, IL-1α, or MMP-1 release was determined. Exposure to 65Jcm-2 of visible light resulted in a significant increase in ROS, IL-1α, and MMP-1 release. Pretreatment with a lotion containing UVA/UVB Sunscreen alone had no effect on reducing the damage from visible light. In contrast, the addition of antioxidants into the same UVA/UVB Sunscreen resulted in a significant reduction in the effects of visible light, reducing ROS, IL-1α, and MMP-1 release by 78%, 82%, and 87%, respectively (Figure 4). The direct effect of the antioxidants alone, without sunscreen, was tested and similarly mitigated the ROS, cytokine, and MMP release induced by visible light (data not shown).
      Figure thumbnail gr4
      Figure 4UVA/UVB sunscreen with an antioxidant reduces reactive oxygen species (ROS), proinflammatory cytokine (IL-1), and matrix metalloprotease (MMP) production induced by visible light. Exposure to visible light significantly increased H2O2, IL-1α, and MMP-1 production, and this increase was not affected by the UVA/UVB sunscreen. In contrast, the UVA/UVB sunscreen containing an antioxidant blend significantly reduced H2O2, IL-1α, and MMP-1 production. An asterisk indicates a significant difference (P<0.05) compared with unexposed skin using analysis of variance with Newman–Keuls post hoc test.

      Measuring free-radical production in the skin from visible light using chemiluminescence

      We next sought to confirm the in vitro ROS results by studying free-radical production on the skin of human subjects. Areas of the skin high in porphyrin content such as the forehead responded to low levels of visible light to induce free-radical production, which could be measured by photon emission or chemiluminescence. A 50Jcm-2 dose at 150mWcm-2 of visible light was able to significantly increase the amount of free radicals by 85.8% over baseline measurements (Figure 5a). The addition of an antioxidant combination comprising Feverfew (T. parthenium) extract, Soy (G. soja) extract, and Gamma Tocopherol to the sunscreen was able to significantly reduce the free radicals by 54% (Figure 5b). These results are consistent with the in vitro ROS results and clearly demonstrate that visible light exposure induces free-radical production by the skin.
      Figure thumbnail gr5
      Figure 5Human subjects were exposed to visible light, and chemiluminescence was measured as a marker of reactive oxygen species. A 50Jcm-2 dose of visible light at 150mWcm-2 significantly increased free-radical production; an asterisk indicates a significant difference (P<0.05) compared with untreated skin (N=40, a). UV sunscreens do not block non-UV longer wavelengths from stimulating free radicals in the skin. The addition of a potent antioxidant combination to the formulation significantly reduced the number of free radicals (b); *P<0.05 compared with sunscreen alone (N=12 per group of sunscreens, N=24 for visible light alone).

      Discussion

      Although visible light comprises about 44% of solar light, few studies have looked at just visible light alone.
      • Fuchs J.
      • Huflejt M.E.
      • Rothfuss L.M.
      • et al.
      Acute effects of near ultraviolet and visible light on the cutaneous antioxidant defense system.
      have observed effects from UVA and visible light on skin, whereas
      • Cho S.
      • Lee M.J.
      • Kim M.S.
      • et al.
      Infrared plus visible light and heat from natural sunlight participate in the expression of MMPs and type I procollagen as well as infiltration of inflammatory cell in human skin in vivo.
      have looked at visible light and IR; however, only
      • Mahmoud B.H.
      • Ruvolo E.
      • Hexsel C.L.
      • et al.
      Impact of long-wavelength UVA and visible light on melanocompentant skin.
      have discussed the potential effects of visible light on skin. Visible light is the only portion of the spectrum visible to the human eye and responsible for general illumination. If we consider that the solar irradiance in the visible range is about 50mWcm-2, the doses used in this study (40–240Jcm-2) would be equivalent to outside midsummer sunlight exposure of approximately 15–90minutes in Houston, TX. The data presented in this paper suggest that visible light can produce some of the same physiological effects as UV, including inflammation, ROS, and matrix-degrading enzymes being generated in the skin.
      It is well known that ROS are produced in skin following UV irradiation (
      • Pathak M.A.
      • Stratton K.
      Free radicals in human skin before and after exposure to light.
      ) and are major mediators of oxidative damage to the skin. Singlet oxygen can be generated from the action of UVA and endogenous photosensitizers, such as porphyrins and flavins (
      • Ravanat J.L.
      • Di Mascio P.
      • Martinez G.R.
      • et al.
      Singlet oxygen induces oxidation of cellular DNA.
      ), which can produce oxidative damage (
      • Pelle E.
      • Huang X.
      • Mammone T.
      • et al.
      Ultraviolet-B-induced oxidative DNA base damage in primary normal human epidermal keratinocytes and inhibition by a hydroxyl radical scavenger.
      ). In the current study, exposing human skin equivalents to increasing doses, 40–180Jcm-2, of visible light resulted in a dose-dependent increase in ROS production similar to UV (Figure 1a). Visible light has been used in light therapy for various conditions such as atopic dermatitis (
      • Byun H.J.
      • Lee H.I.
      • Kim B.
      • et al.
      Full-spectrum light phototherapy for atopic dermatitis.
      ), eczema (
      • Krutmann J.
      • Medve-Koenigs K.
      • Ruzicka T.
      • et al.
      Ultraviolet-free phototherapy.
      ), and also in antimicrobial photochemotherapy (
      • Soukos N.S.
      • Ximenz-Fyvie L.
      • Hamblin M.R.
      • et al.
      Targeted antimicribal photochemotherapy.
      ). These dermatological treatments using visible light tend to be focal applications to the affected lesional skin only to treat the underlying immune response. It is interesting to speculate that the therapeutic effects of visible light for atopic dermatitis and eczema may be mediated by the production of ROS in the skin, similar to the mechanism of psoralen plus UVA treatments for psoriasis. Irradiation of normal human epidermal keratinocytes with UVB can generate a dose-dependent increase in proinflammatory cytokines and expression of MMPs (Figure 1a–c), which was also seen with visible light exposure. IR light has also been shown to be a source of oxidative stress for skin. Exposing human skin fibroblasts to near-IR radiation was shown to induce ROS formation and lead to the subsequent increased expression of MMPs (
      • Schroeder P.
      • Pohl C.
      • Calles C.
      • et al.
      Cellular response to infrared radiation involves retrograde mitochondrial signaling.
      ). Additional studies exposed photoprotected sites of healthy human skin to solar relevant doses of IR radiation. MMP-1 expression in the dermis, but not in the epidermis, was upregulated in 80% of the tested individuals (
      • Schroeder P.
      • Lademann J.
      • Darvin M.E.
      • et al.
      Infrared radiation-induced matrix metalloproteinase in human skin: implications for protection.
      ).
      • Cho S.
      • Lee M.J.
      • Kim M.S.
      • et al.
      Infrared plus visible light and heat from natural sunlight participate in the expression of MMPs and type I procollagen as well as infiltration of inflammatory cell in human skin in vivo.
      were able to show that, in addition to UV, IR plus the visible light spectrum within natural sunlight increases MMP-1 and MMP-9 expression in vivo. Taken together, these findings suggest that although the UV fraction of the solar spectrum is harmful and can cause skin damage, the visible light and the IR light portions of the spectrum may also induce skin damage through to the dermis, which could lead to premature photoaging of skin through the generation of free radicals.
      Exposure to UVR has been shown to activate the EGFR–ERK pathway in keratinocytes (
      • Huang R.P.
      • Wu J.X.
      • Fan Y.
      • et al.
      UV activates growth factor receptors via reactive oxygen intermediates.
      ) and in human skin (
      • Knebel A.
      • Rahmsdorf H.J.
      • Ullrich A.
      • et al.
      Dephosphorylation of receptor tyrosine kinases as target of regulation by radiation, oxidants or alkylating agents.
      ;
      • Katiyar S.K.
      A single physiologic dose of ultraviolet light exposure to human skin in vivo induces phosphorylation of epidermal growth factor receptor.
      ). The rapid phosphorylation of EGFR by UV is known to be reactive oxygen intermediate–mediated (
      • Huang R.P.
      • Wu J.X.
      • Fan Y.
      • et al.
      UV activates growth factor receptors via reactive oxygen intermediates.
      ;
      • Knebel A.
      • Rahmsdorf H.J.
      • Ullrich A.
      • et al.
      Dephosphorylation of receptor tyrosine kinases as target of regulation by radiation, oxidants or alkylating agents.
      ). To investigate the effects of visible light on this pathway, primary human keratinocytes were exposed to UV solar light or dose–response of visible light. Exposure to UV light and visible light (65–180Jcm-2) resulted in increased phosphorylation of the EGFR (detected as phospho-tyrosine) and the downstream marker of cell proliferation p42/44 MAPK, suggesting activation of the EGFR–ERK pathway (Figure 2a and b). To determine whether EGFR is a prerequisite for ERK activation by visible light, human keratinocytes were pretreated with the specific EGFR kinase inhibitor (
      • Osherov N.
      • Levitzki A.
      Epidermal-growth-factor-dependent activation of the src-family kinases.
      ;
      • Ellis A.G.
      • Doherty M.M.
      • Walker F.
      • et al.
      Preclinical analysis of the analinoquinazoline AG1478, a specific small molecule inhibitor of EGF receptor tyrosine kinase.
      ), Tyrphostin (AG1478), before visible light treatment. The presence of Tyrphostin abolished the phosphorylation of ERK, suggesting that EGFR is required for the downstream induction of ERK by visible light (Figure 2c). Activation of the EGFR–ERK pathway has been implicated in UV-induced epidermal hyperplasia (
      • El-Abaseri T.B.
      • Putta S.
      • Hansen L.A.
      • et al.
      Ultraviolet irradiation induces keratinocyte proliferation and epidermal hyperplasia through the activation of the epidermal growth factor receptor.
      ), and also in the activation of MMPs that may accelerate skin aging (
      • Kang K.A.
      • Zhang R.
      • Piao M.J.
      • et al.
      Inhibitory effects of triphlorethol-A on MMP-1 induced by oxidative stress in human keratinocytes via ERK and AP-1 inhibition.
      ). Induction of ERK signaling by visible light might be upstream of the MMP-1 activation shown in human skin equivalents (Figure 1c), and potentially any visible light–induced hyperplasia.
      DNA damage by UVB irradiation results from photochemical reactions consequent to direct absorption of photons by DNA bases. The UV-induced DNA lesions that have been studied in most detail are the cyclobutane pyrimidine dimer and the 6–4 pyrimidine–pyrimidone photoproduct at adjacent pyrimidines (
      • Nakajima S.
      • Lan L.
      • Kanno S.
      • et al.
      UV light-induced DNA damage and tolerance for the survival of nucleotide excision repair-deficient human cells.
      ). Nuclear DNA strand breaks are also readily produced by incubation of keratinocytes with hydrogen peroxide (
      • Armeni T.
      • Battino M.
      • Stronati A.
      • et al.
      Total antioxidant capacity and nuclear DNA damage in keratinocytes after exposure to H2O2.
      ), and hydroxyl radicals can be generated from hydrogen peroxide through Fe2+-mediated Fenton-type reactions (
      • Stewart M.S.
      • Cameron G.S.
      • Pence B.C.
      • et al.
      Antioxidant nutrients protect against UVB-induced oxidative damage to DNA of mouse keratinocytes in culture.
      ). Our studies showed that visible light–irradiated tissues did not induce thymine dimer formation (Figure 3) even at concentrations sufficient to induce significant increases in ROS. In comparison, UV irradiation induced a pronounced DNA damage (Figure 3). These results suggest that, in contrast to UV, visible light photons may not be directly absorbed by DNA bases and therefore may not result in thymine dimer formation. Visible light could contribute to other forms of DNA damage, such as 8-Oxoguanine, which were not measured in the current study. Indeed, exposure of AS52 Chinese hamster cells to a visible light source was reported to produce 8-Oxoguanine DNA damage with a relative maximum between 400 and 450nm (
      • Kielbassa C.
      • Roza L.
      • Epe B.
      • et al.
      Wavelength dependence of oxidative DNA damage induced by UV and visible light.
      ).
      To confirm the in vitro results, we evaluated the effects of visible light on oxidative stress in a clinical model. UVA (320nm–400nnm) exposure has been shown to be a source of oxidative stress in the skin clinically (
      • Ou-Yang H.
      • Stamatas G.
      • Saliou C.
      • et al.
      A chemiluminescence study of UVA-induced oxidative stress in human skin in vivo.
      ), but no studies have examined ROS from visible light alone. To study the effects of visible light (400–700nm) on the skin of subjects, we used a photon detector to measure the rate of photon emission or chemiluminescence. A visible light dose of 50Jcm-2 was able to significantly increase the photon emission on the skin by 85.8% compared with baseline measurements (Figure 5a). A broad UVA/UVB sunscreen was tested for its ability to inhibit free radicals generated from visible light, and this resulted in no change in the amount of photon emission. These data show that the UV sunscreens do not block the longer non-UV wavelengths from stimulating free radicals in the skin. When an antioxidant combination including a Parthenolide-Depleted Feverfew (T. parthenium) extract (
      • Martin K.
      • Sur R.
      • Liebel F.
      • et al.
      Parthenolide-depleted Feverfew (Tanacetum parthenium) protects skin from UV irradiation and external aggression.
      ) was added to the UV sunscreen, a 54% decrease in the amount of generated free radicals from visible light was observed (Figure 5b). These results are consistent with in vitro study that demonstrated that a UVA/UVB sunscreen did not affect the visible light–induced release of ROS, MMP-1, or proinflammatory cytokines from epidermal tissues; however, the addition of a potent antioxidant combination to a UVA/UVB sunscreen significantly reduced the release of all these mediators (Figure 4). These data demonstrate that sunscreens containing UVA/UVB sunfilters alone do not prevent the free-radical production from non-UV longer wavelengths of the solar spectrum.
      When UV contacts the skin, the energy of the photons is absorbed by chromophores, which can mediate the cellular effects of UV exposure. A variety of chromophores, including DNA and some aromatic amino acids, are known to absorb UVR; however, biomolecules that can absorb longer wavelengths such as visible light are not fully understood. A few potential targets for the actions of visible light may include riboflavin, hemoglobin, and bilirubin, all of which show absorbance peaks in the visible range of the spectrum, and are found in the skin. Melanin and β-carotene have been shown to absorb UV light, but can also act as endogenous chromophores for visible light (
      • Anderson R.R.
      • Parrish J.A.
      The optics of human skin.
      ;
      • Mahmoud B.H.
      • Hexsel C.L.
      • Hamzavi I.J.
      • et al.
      Effects of visible light on the skin.
      ). It is also known that the depth of penetration of optical radiation in skin is wavelength dependent, and affected by position and absorption spectrum of the corresponding chromophore in skin (Mahmoud et al., 2008). In the dermis, optical scattering of light is considered to be inversely proportional to wavelength and affects the depth of optical penetration (
      • Anderson R.R.
      • Parrish J.A.
      The optics of human skin.
      ). The approximate depth for penetration of visible light (400–700nm) in a fair-skinned Caucasian individual was estimated to be between 90 and 750μm by
      • Anderson R.R.
      • Parrish J.A.
      The optics of human skin.
      , compared with a depth of 1.5–90μm for UVR (
      • Anderson R.R.
      • Parrish J.A.
      The optics of human skin.
      ). Thus, even though visible light photons are less energetic than UV photons, due to the deeper dermal penetration visible light may still have a substantial effect on skin. Taken together, these results demonstrate that visible light exposure can induce ROS, which can lead to the release of proinflammatory cytokines and MMPs in the skin, similar to the effects of UV, and therefore visible light may contribute to the signs of premature aging in the skin.

      Materials and Methods

      UV and visible light–induced cytokine release

      The UV light source used as a positive control was an 1,000W Oriel solar UV simulator (Oriel, Stratford, CT), equipped with an atmospheric attenuation filter (Schott WG 320, Schott Optical, Elmsford, NY; 1mm thick) and a visible–IR filter (Schott UG 11; 1mm thick). This filtered xenon light source provided a simulated solar UVR spectrum (290–400nm) that was nearly devoid of visible and IR radiation as determined using an IL-1400 Research Radiometer (International Light, Newburyport, MA). The fluence rate of the light sources was 7.52mWcm-2, measured using a calibrated Oriel Thermopile Model 71767.
      The visible light source used was the Fiber-Lite Model 170-D (Dolan-Jenner Industries, Boxborough, MA) with a 150W quartz halogen lamp; a straight 8-mm Dolan-Jenner glass optical fiber was used for irradiation. The source was filtered with three KG5/3-mm Schott glass filters and one GG400/3-mm (Schott Optical Company) was used to filter IR and UVR from the light source, respectively. The light source was characterized spectrally by a calibrated Optronics OL750 spectroradiometer, Optronics Laboratories, Orlando, FL. From the spectroradiometric measurements (from 350 to 1800nm), the light source has 0.14% of UVA (350–400nm), 98.3% of visible light, 1.7% of IRA (700–1400nm), and 0.3% of IRB (Supplementary Figure S1 online). For the doses used in the studies presented in this paper, the contribution from UVA and IR (A and B) are negligible. For the highest visible light dose used in this study, 240Jcm-2, we would have less than 0.34Jcm-2 of UVA and about 4Jcm-2 of IRA being delivered to the irradiated sample. The spectral irradiance of the visible light source is described in
      • Mahmoud B.H.
      • Ruvolo E.
      • Hexsel C.L.
      • et al.
      Impact of long-wavelength UVA and visible light on melanocompentant skin.
      . Skin surface temperature of skin equivalents and media temperature exposed to either UV or visible light sources were measured during exposure periods and did not increase by >1°C.

      Cell culture and skin equivalents

      Normal human epidermal neonatal keratinocytes were obtained from Lifeline Cell Technologies (Frederick, MD) and maintained in Dermalife keratinocyte medium (Lifeline Cell Technologies, Frederick, MD) with supplements. Reconstituted human epidermis (EPI-200-HCF) was purchased from MatTek (Ashland, MA). After reception, epidermal equivalents were incubated with a phenol-free and hydrocortisone-free maintenance medium (MatTek at 37°C for 24h.

      Assessment of UV and visible light–induced ROS and mediator release from reconstituted epidermis

      MatTek epidermal equivalent tissues were topically treated for 45minutes with 5μM of the hydrogen peroxide–sensitive fluorescent probe 5-(and-6)-chloromethyl-2′,7′-dichlorodihydro-fluorescein diacetate, acetyl ester (Invitrogen, Carlsbad, CA). After incubation, the plate was rinsed with warm phosphate-buffered saline to remove excess probe. A volume of 6μl of sunscreen formulation or Feverfew extract (10mgml-1, wt/volt) was topically applied and spread uniformly across the surface of the epidermal equivalent. Tissues were then incubated for 1hour before UV or visible light exposure. Fluorescence was then measured using a CytoFluor Fluorescence Plate Reader (PerSeptive Biosystems, Framingham, MA) set with the following filter combination: excitation at 485nm and emission at 530nm. The tissues were then exposed to UV or visible light, and immediately after exposure the tissues were measured for fluorescence.
      For measurement of cytokines, the equivalents were transferred back to the maintenance medium and incubated at 37°C for 24hours. After 24hours, the medium below each equivalent was collected and analyzed for secreted IL-1α, IL-1 receptor antagonist, IL-6, TNFα, GM-CSF, IL-8, MMP-1, and MMP-9 by ELISA, using commercially available immunoassay multiplex kits (Millipore, Bedford, MA) on a Luminex L100 (Luminex, Austin, TX).

      Western blotting

      Primary keratinocytes grown in 12-well plates were either left untreated, treated with TNFα (100ngml-1) or exposed to UV solar light or multiple doses of visible light for specific periods of time corresponding to the doses. The sham group was left at room temperature for an equivalent amount of time. Post exposure, cells were incubated at 37°C for 20minutes, then washed with phosphate-buffered saline and lysed with radioimmunoprecipitation lysis buffer containing 65mM Tris (pH 7.4), 150mM NaCl, 1mM EDTA (pH 8), 1% nonidet P-40, 0.25% sodium deoxycholate, 50mM NaF, 1mM Na3VO4, 1mM phenylmethylsulfonyl fluoride, and 1 × protease inhibitor cocktail (Sigma, St Louis, MO). Lysates were centrifuged and total protein was estimated in the supernatants using a BCA protein assay kit (Pierce–Thermo Fisher Scientific, Rockford, IL). Protein (30μg) was loaded on SDS-PAGE followed by immunoblotting with the specific antibodies (incubation with primary antibodies diluted 1:1,000 for overnight at 4°C) and detection using the ECL chemiluminescence detection system (Amersham Life Sciences–GE Healthcare, Piscataway, NJ).

      Assessing DNA damage (thymine dimer staining)

      Thymine (T–T) dimer staining of human skin equivalents was performed as described previously (
      • Martin K.
      • Sur R.
      • Liebel F.
      • et al.
      Parthenolide-depleted Feverfew (Tanacetum parthenium) protects skin from UV irradiation and external aggression.
      ). In brief, skin equivalents were either left untreated or exposed to UV or visible light, after which they were immediately fixed in formalin and sent to Paragon BioServices (Baltimore, MD) for T–T dimer staining.

      Chemiluminescence

      Photons emitted from the skin were measured by a red-sensitive, tri-alkali photomultiplier with a 2.5-cm diameter photocathode surface (Electron Tubes, Rockaway, NJ, model 9828SA). The photomultiplier was in a Peltier-cooled housing unit (Electron Tubes, model CDM30) and was routinely cooled to -18 to –20°C to reduce detector noise. The housing unit can be held with one hand as a probe and placed on the skin area to be measured. The surface of the photocathode was protected by a manual shutter (Products for Research, Danvers, Ma, model PR318). The shutter is opened for measurements and closed for instrument background. The distance between the skin and the photocathode surface was approximately 1.5cm. The photomultiplier was supplied with a potential of -1.15kV of high voltage (Brandenburg, Brierley Hill, UK, model 477) with a variation less than ±2V. After going through an amplifier and discriminator (Electronic Tubes, model AD6), the output signals from the photomultiplier were measured in a photon-counting module (Electronic Tubes, model CT1) and recorded by a computer. All measurements were recorded in a positively dark room; otherwise, the background would overwhelm the signal (
      • Ou-Yang H.
      • Stamatas G.
      • Saliou C.
      • et al.
      A chemiluminescence study of UVA-induced oxidative stress in human skin in vivo.
      ). The forehead was used for all the measurements.

      Statistical analysis

      All data are presented as mean±SD. For in vitro studies, a one-way analysis of variance with Newman–Keuls post hoc test was used to determine significance. A value of P<0.05 was considered significant. For chemiluminescence results, data were analyzed using a Student's t-test with significance set at P<0.05.

      ACKNOWLEDGMENTS

      We thank Jean Krutmann for discussions and suggestions on the effect of non-UV light on skin.

      SUPPLEMENTARY MATERIAL

      Supplementary material is linked to the online version of the paper at http://www.nature.com/jid

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