A Cell-Based Evaluation of the Tyrosinase-Mediated Metabolic Activation of Leukoderma-Inducing Phenols, II: The Depletion of Nrf2 Augments the Cytotoxic Effect Evoked by Tyrosinase in Melanogenic Cells.

Chemical leukoderma is a disorder induced by chemicals such as rhododendrol and monobenzone. These compounds possess a p-substituted phenol moiety and undergo oxidation into highly reactive and toxic o-quinone metabolites by tyrosinase. This metabolic activation plays a critical role in the development of leukoderma through the production of damage to melanocytes and immunological responses. This study aimed to develop a simple method for assessing the metabolic activation of leukoderma-inducing phenols without analyzing the metabolite. Although B16BL6 melanoma cells showed insufficient sensitivity to the cytotoxicity assay, the siRNA-mediated knockdown of the transcription factor NRF2 (NFE2L2) repressed the expression of cytoprotective factors, thereby augmenting the cytotoxicity of all six leukoderma-inducing phenols tested in a tyrosinase-dependent manner, indicating enhanced sensitivity to o-quinone metabolites. Additionally, the knockdown of the NRF2-target Slc7a11 elevated the cytotoxicity of three out of the six compounds, indicating the involvement of cystine transport in cellular protection. In contrast, the knockdown or inhibition of the NRF2-target Nqo1 had minimal effects. The same response was induced upon Nrf2 and Slc7a11 knockdown in B16-4A5 cells, albeit with low sensitivity owing to low tyrosinase expression. We conclude that the analysis of tyrosinase-dependent cytotoxicity in Nrf2-depleted B16BL6 cells may serve as a useful strategy for evaluating the metabolic activation of chemicals.