title: Dermal damage promoted by repeated low-level UV-A1 exposure despite tanning response in human skin.
abstract: OBJECTIVE
  Solar UV irradiation causes photoaging, characterized by fragmentation and reduced production of type I collagen fibrils that provide strength to skin. Exposure to UV-B irradiation (280-320 nm) causes these changes by inducing matrix metalloproteinase 1 and suppressing type I collagen synthesis. The role of UV-A irradiation (320-400 nm) in promoting similar molecular alterations is less clear yet important to consider because it is 10 to 100 times more abundant in natural sunlight than UV-B irradiation and penetrates deeper into the dermis than UV-B irradiation. Most (approximately 75%) of solar UV-A irradiation is composed of UV-A1 irradiation (340-400 nm), which is also the primary component of tanning beds.
  
  
  OBJECTIVE
  To evaluate the effects of low levels of UV-A1 irradiation, as might be encountered in daily life, on expression of matrix metalloproteinase 1 and type I procollagen (the precursor of type I collagen).
  
  
  METHODS
  In vivo biochemical analyses were conducted after UV-A1 irradiation of normal human skin at an academic referral center. Participants included 22 healthy individuals without skin disease.
  
  
  METHODS
  Skin pigmentation was measured by a color meter (chromometer) under the L* variable (luminescence), which ranges from 0 (black) to 100 (white). Gene expression in skin samples was assessed by real-time polymerase chain reaction.
  
  
  RESULTS
  Lightly pigmented human skin (L* >65) was exposed up to 4 times (1 exposure/d) to UV-A1 irradiation at a low dose (20 J/cm2), mimicking UV-A levels from strong sun exposure lasting approximately 2 hours. A single exposure to low-dose UV-A1 irradiation darkened skin slightly and did not alter matrix metalloproteinase 1 or type I procollagen gene expression. With repeated low-dose UV-A1 irradiation, skin darkened incrementally with each exposure. Despite this darkening, 2 or more exposures to low-dose UV-A1 irradiation significantly induced matrix metalloproteinase 1 gene expression, which increased progressively with successive exposures. Repeated UV-A1 exposures did not suppress type I procollagen expression.
  
  
  CONCLUSIONS
  A limited number of low-dose UV-A1 exposures, as commonly experienced in daily life, potentially promotes photoaging by affecting breakdown, rather than synthesis, of collagen. Progressive skin darkening in response to repeated low-dose UV-A1 exposures in lightly pigmented individuals does not prevent UV-A1-induced collagenolytic changes. Therefore, for optimal protection against skin damage, sunscreen formulations should filter all UV wavelengths, including UV-A1 irradiation.
authors.affiliation: Department of Dermatology, University of Michigan Medical School, Ann Arbor.
authors.forename: Frank
authors.initials: F
authors.lastname: Wang
authors.affiliation: Department of Dermatology, University of Michigan Medical School, Ann Arbor.
authors.forename: Noah R
authors.initials: NR
authors.lastname: Smith
authors.affiliation: currently a medical student at the University of Michigan Medical School, Ann Arbor.
authors.forename: Bao Anh Patrick
authors.initials: BA
authors.lastname: Tran
authors.affiliation: Department of Dermatology, University of Michigan Medical School, Ann Arbor 3now with Department of Dermatology, Johns Hopkins University, Baltimore, Maryland.
authors.forename: Sewon
authors.initials: S
authors.lastname: Kang
authors.affiliation: Department of Dermatology, University of Michigan Medical School, Ann Arbor.
authors.forename: John J
authors.initials: JJ
authors.lastname: Voorhees
authors.affiliation: Department of Dermatology, University of Michigan Medical School, Ann Arbor.
authors.forename: Gary J
authors.initials: GJ
authors.lastname: Fisher
mesh_terms: D000328:Adult; D024042:Collagen Type I / Q000235:genetics*; D015870:Gene Expression / Q000528:radiation effects*; D006801:Humans; D020781:Matrix Metalloproteinase 1 / Q000235:genetics*; D019278:Matrix Metalloproteinase 3 / Q000235:genetics; D008875:Middle Aged; D012333:RNA, Messenger / Q000378:metabolism; D012867:Skin / Q000378:metabolism / Q000528:radiation effects*; D015595:Skin Aging / Q000528:radiation effects; D055107:Suntan; D014466:Ultraviolet Rays / Q000009:adverse effects*; D015854:Up-Regulation / Q000528:radiation effects; D055815:Young Adult
chemical_list: D024042:Collagen Type I; D012333:RNA, Messenger; D019278:Matrix Metalloproteinase 3; D020781:Matrix Metalloproteinase 1