Inhibition of 6-formylindolo[3,2-b]carbazole metabolism sensitizes keratinocytes to UVA-induced apoptosis: Implications for vemurafenib-induced phototoxicity

Ultraviolet (UV) B irradiation of keratinocytes results in the formation of the tryptophan photoproduct 6-formylindolo[3,2-b]carbazole (FICZ) which is a high-affinity ligand for the aryl hydrocarbon receptor (AHR). The resulting activation of AHR signaling induces the expression of cytochrome P450 (CYP) 1A1 which subsequently metabolizes FICZ. Importantly, FICZ is also a nanomolar photosensitizer for UVA radiation. Here, we assess whether a manipulation of the AHR-CYP1A1 axis in human epidermal keratinocytes affects FICZ/UVA-induced phototoxic effects and whether this interaction might be mechanistically relevant for the phototoxicity of the BRAF inhibitor vemurafenib. Treatment of keratinocytes with an AHR agonist enhanced the CYP1A1-catalyzed metabolism of FICZ and thus prevented UVA photosensitization, whereas an inhibition of either AHR signaling or CYP1A1 enzyme activity resulted in an accumulation of FICZ and a sensitization to UVA-induced oxidative stress and apoptosis. Exposure of keratinocytes to vemurafenib resulted in the same outcome. Specifically, CYP phenotyping revealed that vemurafenib is primarily metabolized by CYP1A1 and to a lesser degree by CYP2J2 and CYP3A4. Hence, vemurafenib sensitized keratinocytes to UVA-induced apoptosis by interfering with the CYP1A1-mediated oxidative metabolism of FICZ. In contrast to this pro-apoptotic effect, a treatment of UVB-damaged keratinocytes with vemurafenib suppressed apoptosis, a process which might contribute to the skin carcinogenicity of the drug. Our results provide insight into the mechanisms responsible for the photosensitizing properties of vemurafenib and deliver novel information about its metabolism which might be relevant regarding potential drug-drug interactions. The data emphasize that the AHR-CYP1A1 axis contributes to the pathogenesis of cutaneous adverse drug reactions.