Flavonoid Nobiletin Exhibits Differential Effects on Cell Viability in Keratinocytes Exposed to UVA versus UVB Radiation

The polymethoxylated flavonoid nobiletin has been shown to suppress inflammatory responses to UVB radiation and to enhance circadian rhythms. Because expression of the core nucleotide excision repair (NER) factor XPA and the rate of removal of UV photoproducts from DNA are regulated by the circadian clock, we investigated whether the beneficial effects of nobiletin in UVB‐exposed cells could be due in part to enhanced NER. Although nobiletin limited UVB‐irradiated human keratinocytes from undergoing cell death, we found that this enhanced survival was not associated with increased NER or XPA expression. Instead, nobiletin reduced initial UV photoproduct formation and promoted a G1 cell cycle arrest. We then examined the implications of this findings for exposures to solar radiation through use of a solar simulated light (SSL) source that contains primarily UVA radiation. In striking contrast to the results obtained with UVB radiation, nobiletin instead sensitized keratinocytes to both the SSL and a more defined UVA radiation source. This enhanced cell death was correlated with a photochemical change in nobiletin absorption spectrum and the production of reactive oxygen species. We conclude that nobiletin is unlikely to be a useful compound for protecting keratinocytes against the harmful effects of solar UV radiation.