Dynamic Regulation of Aryl Hydrocarbon Receptor Function and Activity by Different Stimuli

Abstract: Cells have to sense and adapt to changes in their surrounding environment. Most of the cellular responses to environmental and developmental stimuli are mediated by PAS (Per-ARNT-Sim) proteins. A member of this family of proteins is the aryl hydrocarbon receptor (AhR), a ligandactivated bHLH transcription factor and E3 ubiquitin ligase. Many xenobiotics, in particular the highly toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), bind to and activate the AhR giving this receptor a critical role in mediating xenobiotic-induced toxicity and carcinogenesis. Developmental defects in AhR-/- mice, the strong evolutionary conservation of the AhR and its ubiquitous expression imply a role of this receptor beyond xenobiotic-induced toxicity and xenobiotic metabolism. The physiological function of the AhR has long remained nebulous, but more and more experimental evidence emerges for a role of the AhR in proliferation and differentiation processes and in the maintenance of cellular homeostasis. AhR signaling has mainly been studied upon activation by TCDD, an experimental condition that certainly does not mimic the physiological situation. The photoproduct 6-formylindolo[3,2-b]carbazole which is formed upon irradiation of tryptophan with UVB or visible light, has been suggested as a physiological AhR ligand. As shown in paper I, FICZ is both a high affinity ligand for the AhR and a good substrate for the cytochromes P450 (CYP) family of enzymes and induces its own metabolism suggesting a regulatory circuit between the AhR and CYP enzymes. The identification of FICZ metabolites in human urine indicates the presence of this ligand or its metabolites in biological systems and underlines its physiological relevance. The regulation of AhR function and activity in a normal, xenobiotic-free context was analyzed in paper II using indolocarbazoles as physiological ligands. Due to their rapid metabolism, these ligands evoke a very transient activation of the transcription factor and ubiquitin ligase function of the AhR. On the contrary, AhR stimulation by TCDD results in a sustained transcriptional activation and a persistent down-regulation of AhR and estrogen receptor levels, thereby impairing the receptors responsiveness to new stimulation. This mode of action of TCDD and presumably other non-metabolizable xenobiotics may contribute to their toxicity, whereas dynamic AhR regulation seems to be a prerequisite for normal physiological cell signaling. The regulation of the AhR when activated by UVB was studied in paper III. Upon UVB irradiation, AhR activity and CYP1A1 gene expression were found to be modulated by nuclear factor ?B, probably as part of the cellular stress response to UV. Exposure to UVB and xenobiotics stimulate the formation of protective melanin polymers in melanocytes. As shown in paper IV, these cells enhance both their activity of tyrosinase, the key enzyme for melanin biosynthesis, and their intracellular melanin content upon AhR activation. Melanogenesis was shown to be induced by the AhR through the transcriptional up-regulation of melanogenic genes, without effecting melanocyte proliferation. The results of this study contribute to our knowledge of dioxin toxicity, but first and foremost extend our mechanistic understanding of the regulation and functioning of AhR signaling under physiological conditions. Moreover, a novel function of the AhR in the skin was identified.