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How Much Sun Protection Is Needed?: Are We on the Way to Full-Spectrum Protection?

      The skin-damaging effects of the UV part of solar radiation are well known and therefore are a focus of photobiological research. However, UVR is only a small part of the solar radiation flux that reaches the earth’s surface. In this issue, Liebel et al. shed light on the biological effects of visible solar radiation on human skin.
      For many years, the focus of photobiology research and photo-protective strategies has centered on the UV portion of the solar spectrum of radiation. This is for a good reason, since the high energy of UV radiation induces acute and chronic skin damage already after short time of exposure. Therefore, current sunscreen products are designed to provide high protection in the UVB and UVA ranges of solar radiation. However, only 5% of solar radiation reaching the surface of the earth is within the UV range, whereas about 40–45% is visible light, and ∼50% is in the longer-wavelength infrared (IR) portion. Several groups have reported skin damage due to IR-A radiation (
      • Schroeder P.
      • Lademann J.
      • Darvin M.E.
      • et al.
      Infrared radiation-induced matrix metalloproteinase in human skin: implications for protection.
      ;
      • Zastrow L.
      • Groth N.
      • Klein F.
      • et al.
      The missing link--light-induced (280-1,600nm) free radical formation in human skin.
      ), but little attention has been paid to visible light thus far. In this issue,
      • Liebel F.
      • Kaur S.
      • Ruvolo E.
      • et al.
      Irradiation of skin with visible light induces reactive oxygen species and matrix-degrading enzymes.
      report on oxidative stress induced in human skin in response to visible light. The authors found that irradiation of skin cells with visible light, in doses comparable to 15–90minutes of sunlight exposure, elicited a response similar to that induced by UV radiation, i.e., inflammation, ROS production, and the release of matrix-degrading enzymes.

      The division between UVA1 and visible light is arbitrary

      The historic division between UVA1 and visible light is arbitrary, only determined by the sensitivity of the human eye, which is limited to wavelengths in the solar spectrum that are consequently defined as “visible”. The study of effects of visible light has been hampered in the past by the lack of irradiation devices delivering proper radiation between 400 and 780nm without contamination by UV radiation. However, the current literature shows accumulating evidence that visible light significantly impacts skin physiology (
      • Mahmoud B.H.
      • Hexsel C.L.
      • Hamzavi I.H.
      • et al.
      Effects of visible light on the skin.
      ;
      • Opländer C.
      • Hidding S.
      • Werners F.B.
      • et al.
      Effects of blue light irradiation on human dermal fibroblasts.
      ).

      Visible light induces DNA damage

      Direct damage of DNA by UVB radiation mainly results from absorption of photons by DNA bases. The ensuing DNA lesions are either cyclobutane pyrimidine dimers (CPD) or 6–4 photoproducts. Although high UVA doses have been shown to induce CPDs as well, most of the damage occurs indirectly via ROS generated by endogenous chromophores such as flavins, porphyrins, and bilirubin. This oxidative DNA damage results in base modifications such as 8-oxo-guanosin.
      • Liebel F.
      • Kaur S.
      • Ruvolo E.
      • et al.
      Irradiation of skin with visible light induces reactive oxygen species and matrix-degrading enzymes.
      did not find any thymine dimer formation following visible light irradiation; other forms of DNA damage were not analyzed. However, 8-oxo-guanosin formation was found by
      • Kielbassa C.
      • Roza L.
      • Epe B.
      Wavelength dependence of oxidative DNA damage induced by UV and visible light.
      after irradiation of Chinese hamster cells with visible light. Maximum DNA damage occurred between 400 and 450nm. More research is needed to investigate the exact contribution of visible light to DNA damage, as most of the earlier studies were carried out with mixtures of UV and visible radiation.

      ROS, pro-inflammatory cytokines, and MMP production are induced by visible light

      Figure thumbnail fx1

      Exposure to visible light induces the EGFR–ERK pathway

      UVR is known to activate a plethora of signal transduction pathways, leading to cellular reactions such as DNA repair, growth inhibition, inflammation, and, in the case of melanocytes, to increased melanin production.
      • Liebel F.
      • Kaur S.
      • Ruvolo E.
      • et al.
      Irradiation of skin with visible light induces reactive oxygen species and matrix-degrading enzymes.
      analyzed signal transduction pathways and found that visible light activates the EGFR–ERK pathway. This activation of EGFR–ERK has been demonstrated for UV irradiation (
      • Huang R.P.
      • Wu J.X.
      • Fan Y.
      • et al.
      UV activates growth factor receptors via reactive oxygen intermediates.
      ), and might explain the expression of MMPs in response to visible light, and it points toward a role for visible light in premature skin aging. Given the fact that many different molecules in human skin absorb photons in the visible wavelength range, it is unlikely that only the EGFR–ERK pathway is activated. Here, again, more research is needed to understand the biological effects of visible light.

      Conclusion

      There is increasing evidence that visible light can damage human skin. This effect might seem negligible compared with that of UV-induced skin damage; however, with sunscreen products providing effective protection in the full UV range, the remaining visible light might become a more relevant source of sun-induced oxidative stress.
      • Liebel F.
      • Kaur S.
      • Ruvolo E.
      • et al.
      Irradiation of skin with visible light induces reactive oxygen species and matrix-degrading enzymes.
      showed that sunscreens with effective antioxidants might be a suitable means of reducing visible light–induced ROS and premature skin aging. Future research is required to establish the clinical relevance of these studies.

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