The Uremic Toxin 3-Indoxyl Sulfate Is a Potent Endogenous Agonist for the Human Aryl Hydrocarbon Receptor

Abstract

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor involved in the regulation of multiple cellular pathways, such as xenobiotic metabolism and Th17 cell differentiation. Identification of key physiologically relevant ligand(s) that regulate AHR function remains to be accomplished. Screening of indole metabolites has identified indoxyl-3-sulfate (I3S) as a potent endogenous ligand that selectively activates the human AHR at nanomolar concentrations in primary human hepatocytes, regulating transcription of multiple genes, including: CYP1A1, CYP1A2, CYP1B1, UGT1A1, UGT1A6, IL6, and SAA1. Furthermore, I3S exhibits an ~ 500-fold greater potency in terms of transcriptional activation of the human AHR relative to the mouse AHR in cell lines. Structure-function studies reveal that the sulfate group is important determinant for efficient AHR activation. This is the first phase II enzymatic product identified that can significantly activate the AHR and ligand competition binding assays indicate that I3S is a direct AHR ligand. I3S failed to activate either CAR or PXR. The physiological importance of I3S lies in the fact that it is a key uremic toxin that accumulates to high micromolar concentrations in kidney dialysis patients, but its mechanism of action is unknown. I3S represents the first identified relatively high potency endogenous AHR ligand that plays a key role in human disease progression. These studies provide evidence that the production of I3S can lead to AHR activation and altered drug metabolism. Our results also suggest that prolonged activation of the AHR by I3S may contribute to toxicity observed in kidney dialysis patients and thus represent a possible therapeutic target.The aryl hydrocarbon receptor (AHR) 1 is a ligand-activated transcription factor that belongs to the basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) family of transcription factors, a class of proteins that are considered environmental sensors. Prior to activation, the AHR is located in the cytoplasm of the cell, complexed with two heat-shock protein 90 molecules, X-associated

The Kidney and Uremic Toxin Removal: Glomerulus or Tubule?

“…At present, kynurenine, kynurenic acid, indoleacetic acid, and indoxyl sulfate-all indolic products of tryptophan metabolism-have been shown to bind to the AhR or induce the expression of AhR response genes, such as CYP1A1. 38,[93][94][95][96][97] Moreover, as shown in Table 3, all four metabolites activate the AhR at concentrations either similar to the highest C max determined in dialysis patients (eg, indole acetic acid) or at levels decisively lower than the C max , for instance indoxyl sulfate. To date, the clinical implications of AhR activation in the setting of CKD development and progression remain unclear, however, it has been postulated that uremic solutes might evoke dioxinlike toxicity, 38 leading to suppression of immune responses, induction of carcinogenesis and accelerating tumor growth, and promoting atherosclerosis.…”

Crucial Role of the Aryl Hydrocarbon Receptor (AhR) in Indoxyl Sulfate-Induced Vascular Inflammation

“…In 2010, Schroeder et al demonstrated that the aryl hydrocarbon receptor (AhR) is an endogenous receptor for indoxyl sulfate 24) . AhR is a member of the basic helix -loop -helix Per -Arnt -Sim regulatory protein superfamily.…”

The Aryl Hydrocarbon Receptor is a Critical Regulator of Tissue Factor Stability and an Antithrombotic Target in Uremia

“…14,15 Several endogenous chemicals, including IS, bind to and activate the AHR. 12,16 Therefore, we hypothesized that IS accumulation in CKD patients induces TF through an AHR pathway, thus potentiating thrombosis. Our results demonstrate that the AHR regulates baseline and IS-mediated vSMC TF levels and activity and show the feasibility of targeting this AHR pathway with novel competitive AHR antagonists able to modify CKD-specific thrombotic risk.…”