Octyl methoxycinnamate modulates gene expression and prevents cyclobutane pyrimidine dimer formation but not oxidative DNA damage in UV-exposed human cell lines.

Ultraviolet (UV) radiation from sunlight is considered one of the most important environmental factors affecting humans and has been implicated as the main cause for skin cancer (Ananthaswamy et al., 1997). It is generally thought that ultraviolet B (UVB) (280–320 nm) and to a lesser extent ultraviolet A (UVA) (320–400 nm) are responsible for the sunlight-induced cancers (Armstrong and Kricker, 2001; Cole et al., 1986). The UVA contribution to melanoma skin cancer has been discussed (Mitchell et al., 2007). The cellular effects of UV irradiation include DNA damage, cell cycle arrest, immunological depression, apoptosis, and transcriptional changes. UVB irradiation directly causes bulky DNA adducts such as cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PPs), which are usually repaired by nucleotide excision repair (NER) generally removing bulky DNA adducts (Lehmann, 1995). UV light—in particular UVA but also UVB—mediates oxidative stress indirectly via reactive oxygen species (ROS) (Pelle et al., 2003). The UV-generated ROS can induce DNA single-strand breaks (SSBs), DNA-protein cross-links, and oxidized base derivatives, such as 7,8-dihydro-8-oxoguanine (8-oxoG) (Mitra et al., 1997). Oxidative DNA damage is preferentially removed by base excision repair (BER) (Helbock et al., 1999; Mitra et al., 1997). Sunscreens have become the most popular choice of photoprotection and are recommended in addition to using protective clothing and avoiding the sun (International Agency for Research on Cancer, 2001). Ideally, sunscreens should protect not only against skin cancer but also against effects on the immune system and photoaging of the skin (Elmets and Anderson, 1996; Young and Walker, 2002). There is growing interest in the photostability of sunscreens due to a dramatic increase in their use. The photoinstability of active ingredients in sunscreen has been reported by several studies (Bredholt et al., 1998; Serpone et al., 2002; Tarras-Wahlberg et al., 1999). Upon UV exposure, sunscreens may be degraded to form photoproducts that are potentially toxic. It has been shown that some sunscreens change their spectral performance or may act as photooxidants via generation of free radicals and ROS upon UV exposure (Brezova et al., 2005; Gulston and Knowland, 1999). There are some reports of the protective effect of sunscreens against DNA damage formation in cell cultures (Reinhardt et al., 2003) and in the skin of volunteers (Al Mahroos et al., 2002). The decrease in the efficiency of sunscreens can be caused by different mechanisms: photoisomerization, photodecomposition, and interaction with the formulation or other sunscreen agents (Maier et al., 2001). Therefore, knowledge about sunscreen photostability on cellular processes is important. Octyl methoxycinnamate (OMC) is the most commonly used UVB filter in sunscreens and cosmetics and is listed as a high production volume chemical in the European chemical Substances Information System database (http://ecb.jrc.ec.europa.eu/esis/). Topical application of OMC is well tolerated, with little or negligible skin irritation, allergic contact reaction, and phototoxic effects. However, we have previously reported increased toxicity as a result of breakdown of OMC following UV irradiation (Butt and Christensen, 2000). Such UV-induced molecular breakdown may interfere with cellular processes or induce oxidative damage in human skin. OMC has been shown to degrade into photoproducts when exposed to sunlight, which leads to a decrease in UV absorption efficiency (Butt and Christensen, 2000; Pangnakorn et al., 2007). These photoproducts may have a higher toxicity than OMC itself. Other adverse effects of sunscreens have been suggested, including formation of singlet oxygen and various estrogenic effects after in vitro and in vivo exposure to several UV filters (Allen et al., 1996; Schlumpf et al., 2001). To address the need for nonanimal testing of ingredients in cosmetics, we established an in vitro cellular system based on two human cell lines, the primary skin fibroblast cell line (GM00498) and the breast cancer cell line (MCF-7). We investigated the effect of OMC with and without UV irradiation on the expression of a panel of 17 genes, selected because of their role in DNA damage response pathways, by quantitative real-time PCR (qRT-PCR) and the expression of p53 protein by Western blotting. The photoprotective capacities of OMC against UV-induced DNA damage, particularly CPDs and oxidative DNA damage, were evaluated by measuring DNA damage using the alkaline elution assay. Investigation of the expression of DNA damage response–related genes, along with genotoxicity assays, may provide sensitive biological end points that could be useful for screening and evaluation of agents to be used in sunscreens.