Discovery and biological characterization of 1-(1H-indol-3-yl)-9H-pyrido[3,4-b]indole as an aryl hydrocarbon receptor activator generated by photoactivation of tryptophan by sunlight

Abstract

Activation of the aryl hydrocarbon receptor (AHR) by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is required for AHR dependent transcriptional activation and TCDD toxicity. We previously reported that aqueous tryptophan exposed to sunlight through window glass (aTRP) contains multiple photoproducts, including the well characterized 6-formylindolo[3,2-b]carbazole (FICZ), capable of activating the AHR and inducing CYP1A and CYP1A-mediated enzyme activities. We report here the isolation from aTRP and chemical characterization and synthesis of 1-(1H-indol-3-yl)-9H-pyrido[3,4-b]indole (IPI), a compound previously identified as a natural product of marine ascidia and now shown to be a TRP photoproduct with AHR-inducing properties. IPI, FICZ and TCDD produced equieffective induction of CYP1A-mediated 7-ethoxyresorufin deethylase (EROD) activity in chick embryo primary hepatocytes and mammalian Hepa1c1c7 cells. EROD induction by IPI was markedly curtailed in AHR-defective c35 cells, supporting the AHR dependence of the IPI response. Although IPI had a higher EC50 for EROD induction than FICZ, the much larger amount of IPI than FICZ in aTRP makes IPI a prominent contributor to EROD induction in aTRP. IPI was detected in TRP-containing culture medium under ambient laboratory conditions but not in TRP-free medium, consistent with its production from TRP. Cotreatment of hepatocytes with submaximal EROD-inducing doses of IPI and FICZ or TCDD produced additive increases in EROD without synergistic or inhibitory interactions. IPI and FICZ were readily metabolized by cultured hepatocytes. In addition to increasing CYP1A4 mRNA and EROD, IPI and FICZ decreased hepatocyte phosphoenolpyruvate carboxykinase mRNA expression and glucose output, biological effects associated with TCDD metabolic dysregulation. The findings underscore a role for sunlight in generating AHR-activating bioactive molecules.

Aryl Hydrocarbon Receptor Control of Adaptive Immunity

“…Although initially thought to exert only local AhR-mediated effects in the skin, the influence of these photometabolites may be systemic as FICZ has been found in human urine (Wincent et al, 2009). Finally, FICZ is readily metabolized by CYP1A1, CYP1A2, and CYP1B1 (Wincent et al, 2012), satisfying a proposed criterion for endogenous AhR ligands (Diani-Moore et al, 2011). 5.…”

“…FICZ also influences responses to ultraviolet light, genomic stability, and circadian rhythms, all postulated to be AhR mediated (Wincent et al, 2012). At least one additional tryptophan photometabolite, 1-(1H-indol-3-yl)-9H-pyrido [3,4-b]indole, has been identified and shown to exhibit a similar potency and efficacy as FICZ (Diani-Moore et al, 2011). Although initially thought to exert only local AhR-mediated effects in the skin, the influence of these photometabolites may be systemic as FICZ has been found in human urine (Wincent et al, 2009).…”

Host–microbiome interactions: the aryl hydrocarbon receptor as a critical node in tryptophan metabolites to brain signaling

“…Upon exposure to visible or UV light, Trp can be converted to photoproducts, including FICZ and 1-(1 H-indol-3-yl)-9 H-pyrido[3,4-b]indole (IPI), both of which have been reported for their AhR-inducing properties. 68 AhR activated by FICZ could protect the intestinal epithelial barrier from the destruction caused by tumor necrosis factor-alpha (TNFα)/IFN-γ, 15,69 demonstrating that FICZ may be developed as a potent drug for resistance to intestinal diseases. Recent studies also show that FICZ can be formed through novel light-independent pathways 16 (Figure 4).…”

“…16 Additionally, IPI has an equivalent CYP1Ainducing efficacy to FICZ. 68 IPI-induced 7-ethoxyresorufin demethylase (EROD) activity notably decreased in AhR-defective c35 cells, supporting that the IPI response depends on AhR. 68 However, the AhR-activating efficacy of IPI in vivo and its mechanism are unclear.…”

FICZ, a Tryptophan photoproduct, suppresses pulmonary eosinophilia and Th2-type cytokine production in a mouse model of ovalbumin-induced allergic asthma

“…The rate of conversion from tryptophan to FICZ is about 0.0001% in cells and ex vitro [40,44] and the turnover rate is high with a decrease by 81% after 24 h of incubation [44]. At present, it remains to be seen how much of FICZ is produced in the skin exposed to UV and how relevant the results in this study is to asthma incidence in humans, because of differences in tryptophan metabolism among mammalian species [45,46].…”