Identification of triptolide as a potential aryl hydrocarbon receptor antagonist in human T cells and keratinocytes

In China extracts of the herb Tripterygium wilfordii Hook F. (TwHF) are successfully used to treat psoriasis and other autoimmune and/or inflammatory diseases due to its favorable cost-benefit ratio. Triptolide has turned out to be the active substance of TwHF-extracts and has been shown to exert potent anti-inflammatory and immune-suppressive effects in vitro and in vivo . The immunosuppressive action of triptolide has been generally attributed to suppression of T-lymphocyte activation. Recently, it was found that triptolide inhibited the differentiation of murine CD4+ T cells into Th17 cells and decreased the transcription level of interleukin (IL)-17 mRNA and IL-6-induced phosphorylation of STAT3, a key signaling molecule involved in the development of Th17 cells. The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor best known for mediating the toxicity of dioxin. It was shown that in a CD4+ T-cell lineage of mice AhR expression is restricted to the Th17 cell subset and its ligation results in the production of the Th17 cytokines IL-17 and IL-22. Ligation of AhR by 6-formylindolo[3,2-b]carbazole (FICZ), a tryptophan-derived photoproduct that is thought to be an endogenous agonist with high affinity for the AhR receptor, upregulates the expression of IL-17A, IL-17F and IL-22 in human Th17 cells, as well as induction of genes encoding for the xenobiotic metabolizing cytochrome P450 enzymes such as CYP1A1 and CYP1B1. From the molecular structure of triptolide the hypothesis was generated that the compound could bind to AhR and serve as either agonist or antagonist. For this purpose it was investigated whether triptolide could induce cell death and if the exposure to triptolide could affect the differentiation of naïve human T cells to Th17 cells and FICZ induced CYP1A1 and CYP1B1 transcription in both keratinocytes and naive T cells. The results showed that, comparison of Th17 differentiation in naïve T cells by intracellular staining and ELISA after the addition of FICZ together with different concentrations of triptolide showed strongly decreased IFN-γ, IL-17A and IL-22 production in a dose-dependent manner. Quantitive PCR demonstrated a significant down-regulation of IFN-γ, IL-17A and IL-22 mRNA expression by triptolide in naive T cells. Additionally, triptolide potently down-regulated FICZ-induced CYP1A1 and CYP1B1 mRNA expression in both keratinocytes and naive T cells, which indicates that TP inhibits FICZ-mediated cytochrome P450 induction by transcriptional regulation. In conclusion, triptolide exhibits a high capacity to effectively compete with AhR agonist thus repressing AhR-mediated gene induction and protein expression, and its treatment of various cell-types reveals no species dependency with regard to antagonism. These data suggest that the activity of triptolide as an AhR-antagonist may be linked to its anti-inflammatory and immune-suppressive effects. Since triptolide is successfully used as a therapeutic of autoimmune disorders in China our identification of triptolide as an AhR-antagonist suggests that the AhR could be a therapeutic target of interest.