title: UVA-induced oxidative damage and cytotoxicity depend on the mode of exposure.
abstract: The reciprocity rule (Bunsen-Roscoe law) states that a photochemical reaction is directly proportional to the total energy dose, irrespective of the dose distribution. In photomedicine the validity of this law is usually taken for granted, although the influence of radiation intensity and dose distribution are largely unknown. We have examined in a tissue culture model the effects of fractionated versus single dose exposure to UV from a metal halide source on survival, DNA synthesis, glutathione, and oxidative membrane damage. Exposure to fractionated UVA was followed by an increased rate of cell death compared to single dose exposure, when intervals between fractions where short (10-120 min). Longer intervals had the opposite effect. Corresponding results were obtained for DNA synthesis (BrdU incorporation). The increased cytotoxicity of dose fractionation with short intervals could not be abrogated by non-enzymatic antioxidants (astaxanthin, ascorbic acid, alpha-tocopherol). Fractionated irradiation with short intervals led to higher degree of depletion of glutathione (GSH) and to enhanced formation of thiobarbituric acid reactive substances (TBARS) in comparison to an identical single dose. Long intervals between fractions induced opposite effects. Taken together, these data indicate that immediately after UVA exposure cells are more sensitive to a further oxidative attack making repeated exposure with short intervals more cytotoxic than continuous single dose UVA. This might have implications also for responses to UVA in vivo and further studies will have to extend these findings to the situation in healthy and diseased human skin.
authors.affiliation: Division of Special and Environmental Dermatology, Department of Dermatology, Medical University of Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria.
authors.forename: Helga
authors.initials: H
authors.lastname: Merwald
authors.forename: Gabriele
authors.initials: G
authors.lastname: Klosner
authors.forename: Claudia
authors.initials: C
authors.lastname: Kokesch
authors.forename: Manon
authors.initials: M
authors.lastname: Der-Petrossian
authors.forename: Herbert
authors.initials: H
authors.lastname: Hönigsmann
authors.forename: Franz
authors.initials: F
authors.lastname: Trautinger
mesh_terms: D000975:Antioxidants / Q000494:pharmacology; D002294:Carcinoma, Squamous Cell / Q000378:metabolism* / Q000473:pathology*; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000473:pathology / Q000528:radiation effects; D049109:Cell Proliferation / Q000187:drug effects / Q000528:radiation effects; D002470:Cell Survival / Q000528:radiation effects; D004247:DNA / Q000096:biosynthesis; D004261:DNA Replication / Q000528:radiation effects; D005978:Glutathione / Q000378:metabolism; D006801:Humans; D010084:Oxidation-Reduction / Q000528:radiation effects; D018384:Oxidative Stress / Q000528:radiation effects*; D013057:Spectrum Analysis; D017392:Thiobarbituric Acid Reactive Substances / Q000378:metabolism; D014466:Ultraviolet Rays
chemical_list: D000975:Antioxidants; D017392:Thiobarbituric Acid Reactive Substances; D004247:DNA; D005978:Glutathione