Illumination of human keratinocytes in the presence of the sunscreen ingredient Padimate-O and through an SPF-15 sunscreen reduces direct photodamageto DNA but increases strand breaks

On illumination with simulated sunlight, the WE-absorbing sunscreen chemica l 2-ethylhexyl-4-dimethylaminobenzoate (Padimate-O) generates excited speci es which inflict non-ligatable strand breaks on DNA in vitro and it also be comes mutagenic to yeast in vivo. Padimate-O is known to penetrate human sk in but its effects on human cells are not clear. Here, we first simulate th e sunlight which penetrates human skin and use it to illuminate human kerat inocytes. The DNA damage observed in terms of UV-endonuclease-sensitive sit es (ESS) and direct strand breaks per kilobase (kb) of DNA per joule per sq uare metre agrees well with that predicted from action spectra based on mon ochromatic Light. Using plasmid DNA in vitro, we find a very similar patter n of results. Next, we simulate the spectrum that results when the incident light is first attenuated by a film of sunscreen (SPF-15; 2 mg/cm(2)) cont aining benzophenone-3 (a WA absorber), octyl methoxycinnamate (a WE absorbe r), and Padimate-O. If the sunscreen is not in contact with keratinocytes i t reduces direct DNA damage from sunlight (ESS). However, any Padimate-O in contact with the cells substantially increases indirect damage (strand bre aks) even though the film of sunscreen reduces direct photodamage. We estim ate that applying an SPF-15 sunscreen which contains Padimate-O to human sk in followed by exposure to only 5 minimum erythemal doses (MED) of sunlight could, while suppressing the formation of ESS, increase strand breaks in c ells under the epidermis by at least 75-fold compared to exposure to 1 MED in the absence of sunscreen. (C) 1999 Elsevier Science B.V. All rights rese rved.