ROS have been shown to damage nuclear DNA, cell membranes, and proteins, and initiate lipid peroxidation which can lead to aging, inflammation, cardiovascular diseases, and cancer [13
]. ROS can also alter signal transduction systems that direct gene expression controlling the balance of cell proliferation, differentiation and apoptosis [19
]. Lipid peroxidation produces lipid alkoxy radicals and aldehydes of low molecular weight, such as malondialdehyde and 4-hydroxy-2-nonenal that bind covalently with cellular DNA forming DNA adducts which is associated with cancer development in experimental animals [7
In this paper, we report that photoirradiation of RP in ethanol with light that was a mixture of 53.6% UVB light and 46% UVA light (referred to as UVB light) resulted in the formation of photodecomposition products similar to those formed with a light source that was 98.9% UVA light. The photodecomposition products that were identified following UVB irradiation included 4-keto-RP, 11-ethoxy-12-hydroxy-RP, 13-ethoxy-14-hydroxy-RP, and AR, and the tentatively identified isomeric 15-ethoxy-ARs. Our previous study on photoirradiation of RP with UVA light has shown that the formation of 4-keto-RP, 11-ethoxy-12-hydroxy-RP, 13-ethoxy-14-hydroxy-RP is mediated through a free radical chain reaction, and that the formation of AR is through the ionic photodissociation reaction [7
The photoirradiation of RP with UVB light in the presence of a lipid (methyl linoleate) resulted in the formation of lipid peroxidation products (methyl linoleate hydroperoxides). The formation of the lipid hydroperoxides was inhibited by the free radical scavenger sodium azide.
We have demonstrated that the photodecomposition products formed following irradiation of RP with UVB light are very similar to those formed following photoirradiation of RP with UVA light. In addition, our results strongly suggest that RP, photoexcited by a high output UVB light source (53.6 % UVB light and 46% UVA light), reacts with molecular oxygen to generate both superoxide radical anion and singlet oxygen. Analogously, we have reported that photoexcitation of RP using a high output UVA light source (98.9% UVA light) results in the generation of the same ROS. These results are important for the design and interpretation of long-term biological testing of RP. A variety of light sources are used to perform photocarcinogenicity and photoaging studies. These studies frequently use light sources whose emissions resemble those described for our high output UVA light souces and high output UVB light sources. Our results suggest that, independent of light source, qualitatively and quantitatively similar photodecomposition products and ROS will be formed.
The biological effects caused by photoexcitation of RP are not well understood. Based on our present results and previously reported in vitro
studies on photoexcited RP, we propose a mechanism for product formation and biological consequences following irradiation of RP (). We have previously reported that UVA irradiation of RP, AR and 5,6-epoxy-RP induces DNA damage and cytotoxicity [23
]. We have also demonstrated that treatment of mouse lymphoma cells with RP and subsequent exposure to UVA light resulted in mutations and loss of heterozygosity (LOH), suggesting that photoirradiated RP is phototoxic and photoclastogenic [24
]. These in
vitro studies suggest that exposure of RP to sunlight is a potential health risk. However, one must be cautious in using these results for evaluation of risks associated with exposure to topically applied RP-containing cosmetics and sunlight. Additional factors, such as rates of percutaneous absorption and potential sunscreen effects, must be considered when assessing the effects of topically applied RP on light-induced toxicity. Indeed, Antille et al. [25
] have shown that RP, topically applied to the skin of human volunteers, can reduce acute sensitivity to UV light measured as minimal erythema dose and formation of thymine dimers. Long-term biological tests are warranted to investigate the relative importance of RP’s photoprotective activity and of photodecomposition products and ROS formed after photoexcitation of RP.
The proposed mechanistic pathways initiated by photoirradiation of RP with UVB light leading to generation of RP photodecomposition products, ROS, lipid peroxidation, and DNA damage.