Abstract
Long-term treatment with thiopurines, such as the widely used anticancer, immunosuppressive and anti-inflammatory agent azathioprine, combined with exposure to ultraviolet (UV) radiation is associated with increased oxidative stress, hyperphotosensitivity and high risk for development of aggressive squamous cell carcinomas of the skin. Sulforaphane, an isothiocyanate derived from broccoli, is a potent inducer of endogenous cellular defenses regulated by transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2), including cytoprotective enzymes and glutathione, which in turn act as efficient indirect and direct antioxidants that have long-lasting effects. Treatment with 6-thioguanine, a surrogate for azathioprine, leads to profound sensitization to oxidative stress and glutathione depletion upon exposure to UVA radiation, the damaging effects of which are primarily mediated by generation of reactive oxygen species. The degree of sensitization is greater for irradiation exposures spanning the absorption spectrum of 6-thioguanine, and is dependent on the length of treatment and the level of guanine substitution with 6-thioguanine, suggesting that the 6-thioguanine that is incorporated in genomic DNA is largely responsible for this sensitization. Sulforaphane provides protection against UVA, but not UVB, radiation without affecting the levels of 6-thioguanine incorporation into DNA. The protective effect is lost under conditions of Nrf2 deficiency, implying that it is due to induction of Nrf2-dependent cytoprotective proteins, and that this strategy could provide protection against any potentially photosensitizing drugs that generate electrophilic or reactive oxygen species. Thus, our findings support the development of Nrf2 activators as protectors against drug-mediated photooxidative stress and encourage future clinical trials in populations at high risk for cutaneous photodamage and photocarcinogenesis.