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Original Investigation |

Dermal Damage Promoted by Repeated Low-Level UV-A1 Exposure Despite Tanning Response in Human Skin

Frank Wang, MD1; Noah R. Smith, MD1; Bao Anh Patrick Tran, BA2; Sewon Kang, MD1,3; John J. Voorhees, MD1; Gary J. Fisher, PhD1
[+] Author Affiliations
1Department of Dermatology, University of Michigan Medical School, Ann Arbor
2currently a medical student at the University of Michigan Medical School, Ann Arbor
3now with Department of Dermatology, Johns Hopkins University, Baltimore, Maryland
JAMA Dermatol. 2014;150(4):401-406. doi:10.1001/jamadermatol.2013.8417.
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Importance  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).

Design, Setting, and Participants  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.

Main Outcomes and Measures  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 and Relevance  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.

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Figure 1.
Dose-Dependent Effects of UV-A1 Irradiation on Lightly Pigmented Human Skin

Skin pigmentation was measured using a chromometer under the L* variable (luminescence), which ranges from 0 (black) to 100 (white). Lightly pigmented (L* >65) buttock skin of 10 healthy humans was exposed to the indicated doses of UV-A1 irradiation. A, Changes in skin pigmentation were measured 24 hours after exposure (L* value). B, Skin samples (4 mm) were obtained 24 hours after exposure and were evaluated using real-time polymerase chain reaction to assess gene expression of matrix metalloproteinase 1 (MMP-1), MMP-3, type I procollagen (COL-I), and type III procollagen (COL-III). Data are presented as mean (SE) fold change. mRNA indicates messenger RNA. aP < .05 compared with no UV-A1 irradiation.bP < .05 when comparing response to different UV-A1 doses.

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Figure 2.
Effects of Repeated Daily Exposure of Lightly Pigmented Human Skin to Low-Dose UV-A1 Irradiation

Skin pigmentation was measured using a chromometer under the L* variable (luminescence). Lightly pigmented (L* >65) buttock skin of 12 healthy humans was exposed to low-dose UV-A1 irradiation (20 J/cm2) 1, 2, 3, or 4 times at daily intervals. A, Changes in skin pigmentation were measured 24 hours after each exposure (L* value). B, Skin samples (4 mm) were obtained 24 hours after each exposure and were evaluated using real-time polymerase chain reaction to assess gene expression of matrix metalloproteinase 1 (MMP-1), MMP-3, type I procollagen (COL-I), and type III procollagen (COL-III). Data are presented as mean (SE) fold change. mRNA indicates messenger RNA. aP < .05 compared with no UV-A1 irradiation.bP < .05 when comparing response to different UV-A1 exposures.

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